[SK_BUFF]: Introduce skb_copy_to_linear_data{_offset}

To clearly state the intent of copying to 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@ghostprotocols.net>
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 57a8244..1fb30c3 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -293,7 +293,7 @@
 	if (!skb)
 		return;
 
-	memcpy(skb->data, msg, len);
+	skb_copy_to_linear_data(skb, msg, len);
 	skb->len += len;
 
 	skb_push(skb, sizeof(*udph));