Jesper noted that kmem_cache_destroy() invokes synchronize_rcu() rather than
rcu_barrier() in the SLAB_DESTROY_BY_RCU case, which could result in RCU
callbacks accessing a kmem_cache after it had been destroyed.

Cc: <stable@kernel.org>
Acked-by: NMatt Mackall <mpm@selenic.com>
Reported-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>