test: add msg_zerocopy test

Introduce regression test for msg_zerocopy feature. Send traffic from
one process to another with and without zerocopy.

Evaluate tcp, udp, raw and packet sockets, including variants
- udp: corking and corking with mixed copy/zerocopy calls
- raw: with and without hdrincl
- packet: at both raw and dgram level

Test on both ipv4 and ipv6, optionally with ethtool changes to
disable scatter-gather, tx checksum or tso offload. All of these
can affect zerocopy behavior.

The regression test can be run on a single machine if over a veth
pair. Then skb_orphan_frags_rx must be modified to be identical to
skb_orphan_frags to allow forwarding zerocopy locally.

The msg_zerocopy.sh script will setup the veth pair in network
namespaces and run all tests.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/.gitignore

@@ -1,3 +1,4 @@
+msg_zerocopy
 socket
 psock_fanout
 psock_tpacket

tools/testing/selftests/net/Makefile

@@ -7,7 +7,7 @@ TEST_PROGS := run_netsocktests run_afpackettests test_bpf.sh netdevice.sh
 TEST_GEN_FILES =  socket
 TEST_GEN_FILES += psock_fanout psock_tpacket
 TEST_GEN_FILES += reuseport_bpf reuseport_bpf_cpu reuseport_bpf_numa
-TEST_GEN_FILES += reuseport_dualstack
+TEST_GEN_FILES += reuseport_dualstack msg_zerocopy
 
 include ../lib.mk