RDS: Do not set op_active in r_m_copy_from_user().

Do not allocate sgs for data for 0-length datagrams

Set data.op_active in rds_sendmsg() instead of
rds_message_copy_from_user().
Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/message.c

@@ -330,8 +330,6 @@ int rds_message_copy_from_user(struct rds_message *rm, struct iovec *first_iov,
 			sg++;
 	}
 
-	rm->data.op_active = 1;
-
 out:
 	return ret;
 }

net/rds/send.c

@@ -970,11 +970,14 @@ int rds_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 		goto out;
 	}
 
+	/* Attach data to the rm */
+	if (payload_len) {
 		rm->data.op_sg = rds_message_alloc_sgs(rm, ceil(payload_len, PAGE_SIZE));
-	/* XXX fix this to not allocate memory */
 		ret = rds_message_copy_from_user(rm, msg->msg_iov, payload_len);
 		if (ret)
 			goto out;
+	}
+	rm->data.op_active = 1;
 
 	rm->m_daddr = daddr;