selftests/net: reap zerocopy completions passed up as ancillary data.

PF_RDS sockets pass up cookies for zerocopy completion as ancillary
data. Update msg_zerocopy to reap this information.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 6f3899e602b0f4ec3d62ff385bb805e7109defa4)
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>