slub: Fix kmem_cache_destroy() with SLAB_DESTROY_BY_RCU

slub: Fix kmem_cache_destroy() with SLAB_DESTROY_BY_RCU

commit d76b1590e06a63a3d8697168cd0aabf1c4b3cb3a upstream.

kmem_cache_destroy() should call rcu_barrier() *after* kmem_cache_close() and
*before* sysfs_slab_remove() or risk rcu_free_slab() being called after
kmem_cache is deleted (kfreed).

rmmod nf_conntrack can crash the machine because it has to kmem_cache_destroy()
a SLAB_DESTROY_BY_RCU enabled cache.

Reported-by: Zdenek Kabelac <zdenek.kabelac@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>