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/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
index 9bb63ff..a57ddef 100644
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -1773,8 +1773,7 @@
 	}
 
 	/* Place the datagram payload in the user's iovec. */
-	err = skb_copy_datagram_iovec(skb, sizeof(*dg), msg->msg_iov,
-		payload_len);
+	err = skb_copy_datagram_msg(skb, sizeof(*dg), msg, payload_len);
 	if (err)
 		goto out;