[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/gigaset/usb-gigaset.c b/drivers/isdn/gigaset/usb-gigaset.c
index 2baef34..c8e1c35 100644
--- a/drivers/isdn/gigaset/usb-gigaset.c
+++ b/drivers/isdn/gigaset/usb-gigaset.c
@@ -652,7 +652,7 @@
 	 * transmit data
 	 */
 	count = min(bcs->tx_skb->len, (unsigned) ucs->bulk_out_size);
-	memcpy(ucs->bulk_out_buffer, bcs->tx_skb->data, count);
+	skb_copy_from_linear_data(bcs->tx_skb, ucs->bulk_out_buffer, count);
 	skb_pull(bcs->tx_skb, count);
 	atomic_set(&ucs->busy, 1);
 	gig_dbg(DEBUG_OUTPUT, "write_modem: send %d bytes", count);