dm thin: cleanup noflush_work to use a proper completion

Factor out a pool_work interface that noflush_work makes use of to wait
for and complete work items (in terms of a proper completion struct).
Allows discontinuing the use of a custom completion in terms of atomic_t
and wait_event.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>