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>
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 3f6b735..3d67d1f 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -1225,7 +1225,7 @@
  *	Called once on startup.
  */
 
-static struct packet_type arp_packet_type = {
+static struct packet_type arp_packet_type __read_mostly = {
 	.type =	cpu_to_be16(ETH_P_ARP),
 	.func =	arp_rcv,
 };