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/ipx/af_ipx.c b/net/ipx/af_ipx.c
index 313ef46..a0c7536 100644
--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -1805,8 +1805,7 @@
 		msg->msg_flags |= MSG_TRUNC;
 	}
 
-	rc = skb_copy_datagram_iovec(skb, sizeof(struct ipxhdr), msg->msg_iov,
-				     copied);
+	rc = skb_copy_datagram_msg(skb, sizeof(struct ipxhdr), msg, copied);
 	if (rc)
 		goto out_free;
 	if (skb->tstamp.tv64)