rds: send: mark expected switch fall-through in rds_rm_size

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1465362 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by:  Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/send.c

@@ -902,6 +902,8 @@ static int rds_rm_size(struct msghdr *msg, int num_sgs)
 
 		case RDS_CMSG_ZCOPY_COOKIE:
 			zcopy_cookie = true;
+			/* fall through */
+
 		case RDS_CMSG_RDMA_DEST:
 		case RDS_CMSG_RDMA_MAP:
 			cmsg_groups |= 2;