dm: Remove redundant flush_workqueue() calls

destroy_workqueue() already drains the queue before destroying it, so
there is no need to flush it explicitly.

Remove the redundant flush_workqueue() calls.

This was generated with coccinelle:

@@
expression E;
@@
- 	flush_workqueue(E);
	destroy_workqueue(E);

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>