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/dccp/proto.c

@@ -896,7 +896,7 @@ verify_sock_status:
 		else if (len < skb->len)
 			msg->msg_flags |= MSG_TRUNC;
 
-		if (skb_copy_datagram_iovec(skb, 0, msg->msg_iov, len)) {
+		if (skb_copy_datagram_msg(skb, 0, msg, len)) {
 			/* Exception. Bailout! */
 			len = -EFAULT;
 			break;