net: Add and use skb_copy_datagram_msg() helper.

This encapsulates all of the skb_copy_datagram_iovec() callers
with call argument signature "skb, offset, msghdr->msg_iov, length".

When we move to iov_iters in the networking, the iov_iter object will
sit in the msghdr.

Having a helper like this means there will be less places to touch
during that transformation.

Based upon descriptions and patch from Al Viro.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/llc/af_llc.c

@@ -819,8 +819,7 @@ static int llc_ui_recvmsg(struct kiocb *iocb, struct socket *sock,
 			used = len;
 
 		if (!(flags & MSG_TRUNC)) {
-			int rc = skb_copy_datagram_iovec(skb, offset,
-							 msg->msg_iov, used);
+			int rc = skb_copy_datagram_msg(skb, offset, msg, used);
 			if (rc) {
 				/* Exception. Bailout! */
 				if (!copied)