Slow-work appears to delete its timer as soon as the first user
unregisters, even though other users could be active.  At the same time, it
never seems to delete slow_work_oom_timer.  Arrange for both to happen in
the shutdown path.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>