rxrpc: switch rxrpc_send_data() to iov_iter primitives

Convert skb_add_data() to iov_iter; allows to get rid of the explicit
messing with iovec in its only caller - skb_add_data() will keep advancing
->msg_iter for us, so there's no need to similate that manually.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 85ab7d7..9a8bafe 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2484,19 +2484,18 @@
 }
 
 static inline int skb_add_data(struct sk_buff *skb,
-			       char __user *from, int copy)
+			       struct iov_iter *from, int copy)
 {
 	const int off = skb->len;
 
 	if (skb->ip_summed == CHECKSUM_NONE) {
-		int err = 0;
-		__wsum csum = csum_and_copy_from_user(from, skb_put(skb, copy),
-							    copy, 0, &err);
-		if (!err) {
+		__wsum csum = 0;
+		if (csum_and_copy_from_iter(skb_put(skb, copy), copy,
+					    &csum, from) == copy) {
 			skb->csum = csum_block_add(skb->csum, csum, off);
 			return 0;
 		}
-	} else if (!copy_from_user(skb_put(skb, copy), from, copy))
+	} else if (copy_from_iter(skb_put(skb, copy), copy, from) == copy)
 		return 0;
 
 	__skb_trim(skb, off);