[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/isdn/i4l/isdn_common.c b/drivers/isdn/i4l/isdn_common.c
index 9c926e4..c97330b 100644
--- a/drivers/isdn/i4l/isdn_common.c
+++ b/drivers/isdn/i4l/isdn_common.c
@@ -829,7 +829,7 @@
 				dflag = 0;
 			}
 			count_put = count_pull;
-			memcpy(cp, skb->data, count_put);
+			skb_copy_from_linear_data(skb, cp, count_put);
 			cp += count_put;
 			len -= count_put;
 #ifdef CONFIG_ISDN_AUDIO