block: fix issue with calling blk_stop_queue() from the request_fn handler

When the queue work handler was converted to delayed work, the
stopping was inadvertently made sync as well. Change this back
to being async stop, using __cancel_delayed_work() instead of
cancel_delayed_work().

Reported-by: Jeremy Fitzhardinge <jeremy@goop.org>
Reported-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>