[SK_BUFF]: Introduce skb_copy_from_linear_data{_offset}

To clearly state the intent of copying from linear sk_buffs, _offset being a
overly long variant but interesting for the sake of saving some bytes.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 875da38..34606ef 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -596,7 +596,7 @@
 		 *	Copy the packet header into the new buffer.
 		 */
 
-		memcpy(skb_network_header(skb2), skb->data, hlen);
+		skb_copy_from_linear_data(skb, skb_network_header(skb2), hlen);
 
 		/*
 		 *	Copy a block of the IP datagram.