dm mpath: only flush workqueue when needed

The workqueues are shared by many multipath devices, only flush whole
workqueue when necessary.  Otherwise, we just flush works as needed.

Signed-off-by: wuzhouhui <wuzhouhui14@mails.ucas.ac.cn>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
1 file changed