dm cache: fix a crash due to incorrect work item cancelling

dm cache: fix a crash due to incorrect work item cancelling

The crash can be reproduced by running the lvm2 testsuite test
lvconvert-thin-external-cache.sh for several minutes, e.g.:
  while :; do make check T=shell/lvconvert-thin-external-cache.sh; done

The crash happens in this call chain:
do_waker -> policy_tick -> smq_tick -> end_hotspot_period -> clear_bitset
-> memset -> __memset -- which accesses an invalid pointer in the vmalloc
area.

The work entry on the workqueue is executed even after the bitmap was
freed. The problem is that cancel_delayed_work doesn't wait for the
running work item to finish, so the work item can continue running and
re-submitting itself even after cache_postsuspend. In order to make sure
that the work item won't be running, we must use cancel_delayed_work_sync.

Also, change flush_workqueue to drain_workqueue, so that if some work item
submits itself or another work item, we are properly waiting for both of
them.

Fixes: c6b4fcbad044 ("dm: add cache target")
Cc: stable@vger.kernel.org # v3.9
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>