mtd_blkdevs processes requests one-by-one synchronously from a kthread
and can be easily converted to dequeueing model.  Convert it.

[ Impact: dequeue in-flight request ]
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>