[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/drivers/net/ppp_synctty.c b/drivers/net/ppp_synctty.c
index b6f0e9a..5918fab 100644
--- a/drivers/net/ppp_synctty.c
+++ b/drivers/net/ppp_synctty.c
@@ -594,7 +594,8 @@
 				return NULL;
 			}
 			skb_reserve(npkt,2);
-			memcpy(skb_put(npkt,skb->len), skb->data, skb->len);
+			skb_copy_from_linear_data(skb,
+				      skb_put(npkt, skb->len), skb->len);
 			kfree_skb(skb);
 			skb = npkt;
 		}