[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/xfrm6_tunnel.c b/net/ipv6/xfrm6_tunnel.c
index 60c26c8..fbef782 100644
--- a/net/ipv6/xfrm6_tunnel.c
+++ b/net/ipv6/xfrm6_tunnel.c
@@ -79,7 +79,7 @@
 #define XFRM6_TUNNEL_SPI_MIN	1
 #define XFRM6_TUNNEL_SPI_MAX	0xffffffff
 
-static kmem_cache_t *xfrm6_tunnel_spi_kmem;
+static kmem_cache_t *xfrm6_tunnel_spi_kmem __read_mostly;
 
 #define XFRM6_TUNNEL_SPI_BYADDR_HSIZE 256
 #define XFRM6_TUNNEL_SPI_BYSPI_HSIZE 256