io_uring/opdef: remove 'audit_skip' from SENDMSG_ZC

The msg variants of sending aren't audited separately, so we should not
be setting audit_skip for the zerocopy sendmsg variant either.

Fixes: 493108d9 ("io_uring/net: zerocopy sendmsg")
Reported-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>