drm: Work around busted kthread_join.

drm: Fix error return for kthread_run: error pointer, not null.

drm: Take advantage of kthread interlock to reduce diff.

drm: Rework Linux `kthread' abstraction to avoid race to sleep.

Requires passing in the caller's lock and condvar to kthread_run, but
for the one user that appears not to be an onerous requirement.

drm: Trigger `spurious' wakeup for kthread_stop too.

linux: In kthread_park, wake thread to re-check kthread_shouldpark.

Otherwise, the thread to pakr may be sleeping on some condvar not
noticing it has to notify kthread_park --> deadlock.

linux: Make kthread_park/stop work at IPL_VM.

Needed in order to hold spin locks around it.

linux: Paranoia: Don't read kt_shouldstop unlocked.

drm: linux kthread stubs