dm: stop using WQ_NON_REENTRANT

dm: stop using WQ_NON_REENTRANT

dbf2576e37 ("workqueue: make all workqueues non-reentrant") made
WQ_NON_REENTRANT no-op and the flag is going away.  Remove its usages.

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
(cherry picked from commit 670368a8ddc5df56437444c33b8089afc547c30a)
Signed-off-by: Alex Shi <alex.shi@linaro.org>