net: convert usage of packet_type to read_mostly

Protocols that use packet_type can be __read_mostly section for better
locality. Elminate any unnecessary initializations of NULL.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -1985,9 +1985,8 @@ static const struct proto_ops ax25_proto_ops = {
 /*
  *	Called by socket.c on kernel start up
  */
-static struct packet_type ax25_packet_type = {
+static struct packet_type ax25_packet_type __read_mostly = {
 	.type	=	cpu_to_be16(ETH_P_AX25),
-	.dev	=	NULL,				/* All devices */
 	.func	=	ax25_kiss_rcv,
 };