[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/ipv4/ipmr.c b/net/ipv4/ipmr.c
index dc806b5..9dbf590 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -103,7 +103,7 @@
    In this case data path is free of exclusive locks at all.
  */
 
-static kmem_cache_t *mrt_cachep;
+static kmem_cache_t *mrt_cachep __read_mostly;
 
 static int ip_mr_forward(struct sk_buff *skb, struct mfc_cache *cache, int local);
 static int ipmr_cache_report(struct sk_buff *pkt, vifi_t vifi, int assert);