RDS: make m_rdma_op a member of rds_message

This eliminates a separate memory alloc, although
it is now necessary to add an "r_active" flag, since
it is no longer to use the m_rdma_op pointer as an
indicator of if an rdma op is present.

rdma SGs allocated from rm sg pool.

rds_rm_size also gets bigger. It's a little inefficient to
run through CMSGs twice, but it makes later steps a lot smoother.

Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/rds.h

@@ -316,7 +316,7 @@ struct rds_message {
 	rds_rdma_cookie_t	m_rdma_cookie;
 	struct {
 		struct {
-			struct rds_rdma_op	*m_rdma_op;
+			struct rds_rdma_op	m_rdma_op;
 			struct rds_mr		*m_rdma_mr;
 		} rdma;
 		struct {