[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/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
index c58a6f0..2407a70 100644
--- a/net/xfrm/xfrm_input.c
+++ b/net/xfrm/xfrm_input.c
@@ -12,7 +12,7 @@
 #include <net/ip.h>
 #include <net/xfrm.h>
 
-static kmem_cache_t *secpath_cachep;
+static kmem_cache_t *secpath_cachep __read_mostly;
 
 void __secpath_destroy(struct sec_path *sp)
 {