dm-verity: fix prefetch-vs-suspend race

There's a possible race condition in dm-verity - the prefetch work item
may race with suspend and it is possible that prefetch continues to run
while the device is suspended. Fix this by calling flush_workqueue and
dm_bufio_client_reset in the postsuspend hook.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org