[CIFS] WARN_ON_ONCE if kernel_sendmsg() returns -ENOSPC

kernel_sendmsg() is less likely to return -ENOSPC and it might be
a bug to do so. However, in the past there might have been cases
where a -ENOSPC was returned from a low level driver.

Add a WARN_ON_ONCE() to ensure that it is safe to assume that -ENOSPC
is no longer returned. This -ENOSPC specific handling will be removed
once we are sure it is no longer returned.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/transport.c

@@ -183,6 +183,12 @@ smb_send_kvec(struct TCP_Server_Info *server, struct kvec *iov, size_t n_vec,
 		rc = kernel_sendmsg(ssocket, &smb_msg, &iov[first_vec],
 				    n_vec - first_vec, remaining);
 		if (rc == -ENOSPC || rc == -EAGAIN) {
+			/*
+			 * Catch if a low level driver returns -ENOSPC. This
+			 * WARN_ON will be removed by 3.10 if no one reports
+			 * seeing this.
+			 */
+			WARN_ON_ONCE(rc == -ENOSPC);
 			i++;
 			if (i >= 14 || (!server->noblocksnd && (i > 2))) {
 				cERROR(1, "sends on sock %p stuck for 15 "