rhashtable: Remove unnecessary wmb for future_tbl

The patch 9497df88ab5567daa001829051c5f87161a81ff0 ("rhashtable:
Fix reader/rehash race") added a pair of barriers.  In fact the
wmb is superfluous because every subsequent write to the old or
new hash table uses rcu_assign_pointer, which itself carriers a
full barrier prior to the assignment.

Therefore we may remove the explicit wmb.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/rhashtable.c

@@ -231,9 +231,6 @@ static int rhashtable_rehash_attach(struct rhashtable *ht,
 	 */
 	rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
 
-	/* Ensure the new table is visible to readers. */
-	smp_wmb();
-
 	spin_unlock_bh(old_tbl->locks);
 
 	return 0;