[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/tun.c b/drivers/net/tun.c
index 288d855..4d46159 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -386,8 +386,8 @@
 		 *   - we are multicast promiscous.
 		 *   - we belong to the multicast group.
 		 */
-		memcpy(addr, skb->data,
-		       min_t(size_t, sizeof addr, skb->len));
+		skb_copy_from_linear_data(skb, addr, min_t(size_t, sizeof addr,
+								   skb->len));
 		bit_nr = ether_crc(sizeof addr, addr) >> 26;
 		if ((tun->if_flags & IFF_PROMISC) ||
 				memcmp(addr, tun->dev_addr, sizeof addr) == 0 ||