drm/xe/vm: prevent UAF in rebind_work_func()

We flush the rebind worker during the vm close phase, however in places
like preempt_fence_work_func() we seem to queue the rebind worker
without first checking if the vm has already been closed.  The concern
here is the vm being closed with the worker flushed, but then being
rearmed later, which looks like potential uaf, since there is no actual
refcounting to track the queued worker. We can't take the vm->lock here
in preempt_rebind_work_func() to first check if the vm is closed since
that will deadlock, so instead flush the worker again when the vm
refcount reaches zero.

v2:
 - Grabbing vm->lock in the preempt worker creates a deadlock, so
   checking the closed state is tricky. Instead flush the worker when
   the refcount reaches zero. It should be impossible to queue the
   preempt worker without already holding vm ref.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1676
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1591
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1364
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1304
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1249
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240423074721.119633-4-matthew.auld@intel.com

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