RxRPC: Don't call skb_add_data() if there's no data to copy

Don't call skb_add_data() in rxrpc_send_data() if there's no data to copy and
also skip the calculations associated with it in such a case.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/ar-output.c

@@ -549,8 +549,6 @@ static int rxrpc_send_data(struct kiocb *iocb,
 	if (len > iov_iter_count(&msg->msg_iter))
 		len = iov_iter_count(&msg->msg_iter);
 	do {
-		int copy;
-
 		if (!skb) {
 			size_t size, chunk, max, space;
 
@@ -616,7 +614,8 @@ static int rxrpc_send_data(struct kiocb *iocb,
 		sp = rxrpc_skb(skb);
 
 		/* append next segment of data to the current buffer */
-		copy = skb_tailroom(skb);
+		if (len > 0) {
+			int copy = skb_tailroom(skb);
 			ASSERTCMP(copy, >, 0);
 			if (copy > len)
 				copy = len;
@@ -633,6 +632,7 @@ static int rxrpc_send_data(struct kiocb *iocb,
 			copied += copy;
 
 			len -= copy;
+		}
 
 		/* check for the far side aborting the call or a network error
 		 * occurring */