dm thin: Add missing destroy_work_on_stack()

This commit add missed destroy_work_on_stack() operations for pw->worker in
pool_work_wait().

Fixes: e7a3e871 ("dm thin: cleanup noflush_work to use a proper completion")
Cc: stable@vger.kernel.org
Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>