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/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index c373a9a..21894df 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -424,7 +424,7 @@
 		msg->msg_flags |= MSG_TRUNC;
 		copied = len;
 	}
-	err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+	err = skb_copy_datagram_msg(skb, 0, msg, copied);
 	if (err)
 		goto out_free_skb;
 
diff --git a/net/ipv4/ping.c b/net/ipv4/ping.c
index 57f7c980..736236c 100644
--- a/net/ipv4/ping.c
+++ b/net/ipv4/ping.c
@@ -875,7 +875,7 @@
 	}
 
 	/* Don't bother checking the checksum */
-	err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+	err = skb_copy_datagram_msg(skb, 0, msg, copied);
 	if (err)
 		goto done;
 
diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
index 739db31..ee8fa4b 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -718,7 +718,7 @@
 		copied = len;
 	}
 
-	err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+	err = skb_copy_datagram_msg(skb, 0, msg, copied);
 	if (err)
 		goto done;
 
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 39ec0c3..c239f47 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1377,7 +1377,7 @@
 	/* XXX -- need to support SO_PEEK_OFF */
 
 	skb_queue_walk(&sk->sk_write_queue, skb) {
-		err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, skb->len);
+		err = skb_copy_datagram_msg(skb, 0, msg, skb->len);
 		if (err)
 			break;
 
@@ -1833,8 +1833,7 @@
 		}
 
 		if (!(flags & MSG_TRUNC)) {
-			err = skb_copy_datagram_iovec(skb, offset,
-						      msg->msg_iov, used);
+			err = skb_copy_datagram_msg(skb, offset, msg, used);
 			if (err) {
 				/* Exception. Bailout! */
 				if (!copied)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 3f001db..df19027 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1281,8 +1281,8 @@
 	}
 
 	if (skb_csum_unnecessary(skb))
-		err = skb_copy_datagram_iovec(skb, sizeof(struct udphdr),
-					      msg->msg_iov, copied);
+		err = skb_copy_datagram_msg(skb, sizeof(struct udphdr),
+					    msg, copied);
 	else {
 		err = skb_copy_and_csum_datagram_iovec(skb,
 						       sizeof(struct udphdr),