[fast-reboot-dump] Fix exception in struct.pack (sonic-net#1309)

This commit fix the exception thrown by struct.pack when attempting to pack a unicode string.
The script ```fast-reboot-dump.py``` will throw an exception in python3 because ```struct.pack``` requires ```bytes``` for ```s```.
```
Traceback: Traceback (most recent call last):
#12  File "/usr/local/bin/fast-reboot-dump.py", line 299, in <module>
#12    res = main()
#12  File "/usr/local/bin/fast-reboot-dump.py", line 292, in main
#12    send_garp_nd(neighbor_entries, map_mac_ip_per_vlan)
#12  File "/usr/local/bin/fast-reboot-dump.py", line 221, in send_garp_nd
#12    src_ip_addrs = {vlan_name:get_iface_ip_addr(vlan_name) for vlan_name,_,_ in neighbor_entries}
#12  File "/usr/local/bin/fast-reboot-dump.py", line 221, in <dictcomp>
#12    src_ip_addrs = {vlan_name:get_iface_ip_addr(vlan_name) for vlan_name,_,_ in neighbor_entries}
#12  File "/usr/local/bin/fast-reboot-dump.py", line 195, in get_iface_ip_addr
#12    return get_if(iff, SIOCGIFADDR)[20:24]
#12  File "/usr/local/bin/fast-reboot-dump.py", line 185, in get_if
#12    ifreq = ioctl(s, cmd, struct.pack("16s16x",iff))
#12  struct.error: argument for 's' must be a bytes object
```

Signed-off-by: bingwang <bingwang@microsoft.com>

scripts/fast-reboot-dump.py

@@ -182,7 +182,7 @@ def generate_fdb_entries(filename):
 
 def get_if(iff, cmd):
     s = socket.socket()
-    ifreq = ioctl(s, cmd, struct.pack("16s16x",iff))
+    ifreq = ioctl(s, cmd, struct.pack("16s16x",bytes(iff.encode())))
     s.close()
     return ifreq