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/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index b704a93..c559bcd 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -208,7 +208,7 @@
 	else if (len < skb->len)
 		msg->msg_flags |= MSG_TRUNC;
 
-	err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, len);
+	err = skb_copy_datagram_msg(skb, 0, msg, len);
 	if (likely(err == 0))
 		err = len;