[snmpd] Fixed snmpd memory leak issue (#11812)

#### Why I did it
This fixed memory leak in ETHERLIKE-MIB. The fix is not part of net-snmp(5.7.3 version). This PR includes the patch to fix memory leak issue.

```
ke->name in stdup-ed at line 297: n->name = strdup(RTA_DATA(tb[IFLA_IFNAME]));
```
#### How I did it
patched the fix.
[net-snmp] upstream fix link -> [snmpd}upstream link](net-snmp/net-snmp@ed4e48b)


#### How to verify it
**Before The fix**

used valgrind to find memory leak.
```
root@lnos-x1-a-csw06:/# grep "definitely lost" valgrind-out.txt
==493== 4 bytes in 1 blocks are definitely lost in loss record 1 of 333
==493== 16 bytes in 1 blocks are definitely lost in loss record 25 of 333
==493== 757 bytes in 71 blocks are definitely lost in loss record 214 of 333
==493== 1,168 (32 direct, 1,136 indirect) bytes in 1 blocks are definitely lost in loss record 293 of 333
==493== 1,168 (32 direct, 1,136 indirect) bytes in 1 blocks are definitely lost in loss record 294 of 333
==493== 1,168 (32 direct, 1,136 indirect) bytes in 1 blocks are definitely lost in loss record 295 of 333
==493== 1,168 (32 direct, 1,136 indirect) bytes in 1 blocks are definitely lost in loss record 296 of 333
==493==    definitely lost: 905 bytes in 77 blocks
```
_we can see  the memory leak see in stack trace._
-> dot3stats_linux -> get_nlmsg -> strdup
https://github.com/net-snmp/net-snmp/blob/v5.7.3/agent/mibgroup/etherlike-mib/data_access/dot3stats_linux.c
https://github.com/net-snmp/net-snmp/blob/v5.7.3/agent/mibgroup/etherlike-mib/data_access/dot3stats_linux.c#L277

```
n = malloc(sizeof(*n));
    memset(n, 0, sizeof(*n));

    n->ifindex = ifi->ifi_index;
    n->name = strdup(RTA_DATA(tb[IFLA_IFNAME]));
    memcpy(&n->stats, RTA_DATA(tb[IFLA_STATS]), sizeof(n->stats));
    n->next = kern_db;
    kern_db = n;
    return 0;
```
we were not freeing space for EtherLike-MIB.AS interface mib queries were getting increased, we see memory increment.

```
        kern_db = ke->next;
        free(ke);
```
https://github.com/net-snmp/net-snmp/blob/v5.7.3/agent/mibgroup/etherlike-mib/data_access/dot3stats_linux.c#L467

```
==55== 757 bytes in 71 blocks are definitely lost in loss record 186 of 299
==55==    at 0x483577F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==55==    by 0x4EB6E49: strdup (strdup.c:42)
==55==    by 0x493F278: get_nlmsg (dot3stats_linux.c:299)
==55==    by 0x493F529: rtnl_dump_filter_l.constprop.3 (dot3stats_linux.c:370)
==55==    by 0x493FD7A: rtnl_dump_filter (dot3stats_linux.c:401)
==55==    by 0x493FD7A: _dot3Stats_netlink_get_errorcntrs (dot3stats_linux.c:424)
==55==    by 0x494009F: interface_dot3stats_get_errorcounters (dot3stats_linux.c:530)
==55==    by 0x48F6FDA: dot3StatsTable_container_load (dot3StatsTable_data_access.c:330)
==55==    by 0x485E76B: _cache_load (cache_handler.c:700)
==55==    by 0x485FA37: netsnmp_cache_helper_handler (cache_handler.c:638)
==55==    by 0x48720BC: netsnmp_call_handler (agent_handler.c:526)
==55==    by 0x48720BC: netsnmp_call_next_handler (agent_handler.c:640)
==55==    by 0x4865F75: table_helper_handler (table.c:717)
==55==    by 0x4871B66: netsnmp_call_handler (agent_handler.c:526)
==55==    by 0x4871B66: netsnmp_call_handlers (agent_handler.c:611)

757 bytes in 71 blocks are definitely lost in loss record 214 of 333
==493==    at 0x483577F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==493==    by 0x4EB6E49: strdup (strdup.c:42)
==493==    by 0x493F278: ??? (in /usr/lib/x86_64-linux-gnu/libnetsnmpmibs.so.30.0.3)
==493==    by 0x493F529: ??? (in /usr/lib/x86_64-linux-gnu/libnetsnmpmibs.so.30.0.3)
==493==    by 0x493FD7A: _dot3Stats_netlink_get_errorcntrs (in /usr/lib/x86_64-linux-gnu/libnetsnmpmibs.so.30.0.3)
==493==    by 0x494009F: interface_dot3stats_get_errorcounters (in /usr/lib/x86_64-linux-gnu/libnetsnmpmibs.so.30.0.3)
==493==    by 0x48F6FDA: dot3StatsTable_container_load (in /usr/lib/x86_64-linux-gnu/libnetsnmpmibs.so.30.0.3)
==493==    by 0x485E76B: _cache_load (cache_handler.c:700)
==493==    by 0x485FA37: netsnmp_cache_helper_handler (cache_handler.c:638)
==493==    by 0x48720BC: netsnmp_call_handler (agent_handler.c:526)
==493==    by 0x48720BC: netsnmp_call_next_handler (agent_handler.c:640)
==493==    by 0x4865F75: table_helper_handler (table.c:717)
==493==    by 0x4871B66: netsnmp_call_handler (agent_handler.c:526)
==493==    by 0x4871B66: netsnmp_call_handlers (agent_handler.c:611)
```
```
**After The fix**
no  memory leak in valgrind stack trace related to etherlike MIB.
```

src/snmpd/patch-5.7.3+dfsg/0022-etherlike-mib-memleak.patch

@@ -0,0 +1,9 @@
+--- net-snmp-5.7.3+dfsg/agent/mibgroup/etherlike-mib/data_access/dot3stats_linux.c.org	2020-09-03 19:09:26.241863000 -0700
++++ net-snmp-5.7.3+dfsg/agent/mibgroup/etherlike-mib/data_access/dot3stats_linux.c	2020-09-03 19:24:27.054098000 -0700
+@@ -465,6 +465,7 @@
+             done = 1;
+         }
+         kern_db = ke->next;
++        free(ke->name);
+         free(ke);
+     }

src/snmpd/patch-5.7.3+dfsg/series

@@ -4,3 +4,4 @@
 0007-Linux-VRF-5.7.3-Support.patch
 0008-Enable-macro-DEB_BUILD_ARCH_OS-in-order-to-build-ipv.patch
 cross-compile-changes.patch
+0022-etherlike-mib-memleak.patch