sock: Fix SO_ZEROCOPY switch case

Fix the SO_ZEROCOPY switch case on sock_setsockopt() avoiding the
ret values to be overwritten by the one set on the default case.

Fixes: 28190752

 ("sock: permit SO_ZEROCOPY on PF_RDS socket")
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>