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/tipc/socket.c b/net/tipc/socket.c
index ad8a1a1..591bbfa 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1372,8 +1372,7 @@
 			sz = buf_len;
 			m->msg_flags |= MSG_TRUNC;
 		}
-		res = skb_copy_datagram_iovec(buf, msg_hdr_sz(msg),
-					      m->msg_iov, sz);
+		res = skb_copy_datagram_msg(buf, msg_hdr_sz(msg), m, sz);
 		if (res)
 			goto exit;
 		res = sz;
@@ -1473,8 +1472,8 @@
 		needed = (buf_len - sz_copied);
 		sz_to_copy = (sz <= needed) ? sz : needed;
 
-		res = skb_copy_datagram_iovec(buf, msg_hdr_sz(msg) + offset,
-					      m->msg_iov, sz_to_copy);
+		res = skb_copy_datagram_msg(buf, msg_hdr_sz(msg) + offset,
+					    m, sz_to_copy);
 		if (res)
 			goto exit;