dm: run queue on re-queue

dm: run queue on re-queue

Without kicking queue, requeued request may stay forever in
the queue if there are no other I/O activities to the device.

The original error had been in v2.6.39 with commit c69163c6522d
("block: remove per-queue plugging"), which replaced conditional
plugging by periodic runqueue.

Commit 567cc26c818f in v4.1-rc1 removed the periodic runqueue
and the problem started to manifest.

Fixes: 567cc26c818f ("dm: don't schedule delayed run of the queue if nothing to do")
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>