[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/skbuff.c b/net/core/skbuff.c
index 17c6bb5..331d3ef 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1129,7 +1129,7 @@
 	if ((copy = start - offset) > 0) {
 		if (copy > len)
 			copy = len;
-		memcpy(skb->data + offset, from, copy);
+		skb_copy_to_linear_data_offset(skb, offset, from, copy);
 		if ((len -= copy) == 0)
 			return 0;
 		offset += copy;