Add MSG_ZEROCOPY tests

[jwbensley]

https://www.kernel.org/doc/html/latest/networking/msg_zerocopy.html

The documention uses send(), can this also be used with sendmsg()?

[jwbensley]

From the documentation:

The change to send (or sendto, sendmsg, sendmmsg) itself is trivial. Pass the new flag.

Also:

Copy avoidance is not a free lunch. As implemented, with page pinning, it replaces per byte copy cost with page accounting and completion notification overhead. As a result, MSG_ZEROCOPY is generally only effective at writes over around 10 KB.

[jwbensley]

For documentation:

https://kernelnewbies.org/LinuxChanges#Linux_4.14.Zero-copy_from_user_memory_to_sockets