[NET]: use __read_mostly on kmem_cache_t , DEFINE_SNMP_STAT pointers

This patch puts mostly read only data in the right section
(read_mostly), to help sharing of these data between CPUS without
memory ping pongs.

On one of my production machine, tcp_statistics was sitting in a
heavily modified cache line, so *every* SNMP update had to force a
reload.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index ff685f2..5176fc6 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -67,7 +67,7 @@
 #include <asm/uaccess.h>
 #include <asm/system.h>
 
-DEFINE_SNMP_STAT(struct icmpv6_mib, icmpv6_statistics);
+DEFINE_SNMP_STAT(struct icmpv6_mib, icmpv6_statistics) __read_mostly;
 
 /*
  *	The ICMP socket(s). This is the most convenient way to flow control