HEADS UP: destroy_dev_sched() KPI in the tree

[Kostik Belousov]

Today, I got re@ permission and committed the destroy_dev_sched() KPI
into the tree. As suggested by Nate Lawson, destroy_dev() tries to
auto-detect "devdrn" deadlock condition and transforms destroy_dev()
from d_close() csw method into destroy_dev_sched(). I together with
Peter Holm tried to test the change as thoroughly as possible.

The obvious problematic area are the races between device driver
module unload and destroy_dev_sched(). At least smb(4) definitely
has that race that shall be worked around by draining events with
drain_dev_clone_events() and destroy_dev_drain(&cdevsw); see snp(4)
change for example.

Please, report issues caused by the patch to the list with me Cc:ed.
Change of autodetection of deadlock in destroy_dev() was made in separate
commit for ease of reverting.
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