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/x25/af_x25.c

@@ -1608,7 +1608,7 @@ static const struct proto_ops SOCKOPS_WRAPPED(x25_proto_ops) = {
 
 SOCKOPS_WRAP(x25_proto, AF_X25);
 
-static struct packet_type x25_packet_type = {
+static struct packet_type x25_packet_type __read_mostly = {
 	.type =	cpu_to_be16(ETH_P_X25),
 	.func =	x25_lapb_receive_frame,
 };