[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>

net/decnet/dn_table.c

@@ -79,7 +79,7 @@ for( ; ((f) = *(fp)) != NULL && dn_key_eq((f)->fn_key, (key)); (fp) = &(f)->fn_n
 static DEFINE_RWLOCK(dn_fib_tables_lock);
 struct dn_fib_table *dn_fib_tables[RT_TABLE_MAX + 1];
 
-static kmem_cache_t *dn_hash_kmem;
+static kmem_cache_t *dn_hash_kmem __read_mostly;
 static int dn_fib_hash_zombies;
 
 static inline dn_fib_idx_t dn_hash(dn_fib_key_t key, struct dn_zone *dz)