blk_cleanup_queue() calls elevator_exit() and after this, we can't
touch the elevator without oopsing.  __elv_next_request() must check
for this state because in the refcounted queue model, we can still
call it after blk_cleanup_queue() has been called.

This was reported as causing an oops attributable to scsi.
Signed-off-by: NJames Bottomley <James.Bottomley@suse.de>
Cc: stable@kernel.org
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>