af_packet: use sizeof instead of constant in spkt_device

This small patch removes access to the last element of the spkt_device
array through a constant. Instead, it is accessed by sizeof() to respect
possible changes in if_packet.h.
Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1475,7 +1475,7 @@ static int packet_sendmsg_spkt(struct kiocb *iocb, struct socket *sock,
 	 *	Find the device first to size check it
 	 */
 
-	saddr->spkt_device[13] = 0;
+	saddr->spkt_device[sizeof(saddr->spkt_device) - 1] = 0;
 retry:
 	rcu_read_lock();
 	dev = dev_get_by_name_rcu(sock_net(sk), saddr->spkt_device);