[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/ipv4/ipcomp.c b/net/ipv4/ipcomp.c
index ba348b1..ab86137 100644
--- a/net/ipv4/ipcomp.c
+++ b/net/ipv4/ipcomp.c
@@ -66,7 +66,7 @@
 
 	skb->truesize += dlen - plen;
 	__skb_put(skb, dlen - plen);
-	memcpy(skb->data, scratch, dlen);
+	skb_copy_to_linear_data(skb, scratch, dlen);
 out:
 	put_cpu();
 	return err;