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/rxrpc/ar-recvmsg.c b/net/rxrpc/ar-recvmsg.c
index e9aaa65..4575485 100644
--- a/net/rxrpc/ar-recvmsg.c
+++ b/net/rxrpc/ar-recvmsg.c
@@ -180,7 +180,7 @@
 		if (copy > len - copied)
 			copy = len - copied;
 
-		ret = skb_copy_datagram_iovec(skb, offset, msg->msg_iov, copy);
+		ret = skb_copy_datagram_msg(skb, offset, msg, copy);
 
 		if (ret < 0)
 			goto copy_error;