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>
diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c
index 075a0fb..0cbcf98 100644
--- a/net/ipv6/raw.c
+++ b/net/ipv6/raw.c
@@ -486,11 +486,11 @@
 	}
 
 	if (skb_csum_unnecessary(skb)) {
-		err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+		err = skb_copy_datagram_msg(skb, 0, msg, copied);
 	} else if (msg->msg_flags&MSG_TRUNC) {
 		if (__skb_checksum_complete(skb))
 			goto csum_copy_err;
-		err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+		err = skb_copy_datagram_msg(skb, 0, msg, copied);
 	} else {
 		err = skb_copy_and_csum_datagram_iovec(skb, 0, msg->msg_iov);
 		if (err == -EINVAL)