Explain this SLAB_DESTROY_BY_RCU thing...

[hugh@veritas.com: add a pointer to comment in mm/slab.c]
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: NJens Axboe <jens.axboe@oracle.com>
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NChristoph Lameter <cl@linux-foundation.org>
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>