remoteproc: Prevent crash handling to race with rproc_del()

In the Linux kernel, the following vulnerability has been resolved:

remoteproc: Prevent crash handling to race with rproc_del()

There's no synchronization between rproc_crash_handler_work() and
rproc_del(), as such it's possible for a driver to be removed while
crash-handler work is scheduled, or even executing - resulting in
use-after-free issues.

To avoid this the scheduled work need to be cancelled and synchronized
against before the removal proceeds.

In order to ensure that this doesn't race with the reporting, and
thereby scheduling new work, a "deleting" flag is introduced. This is
similar to the RPROC_DELETE state that was introduced to ensure that
"start" didn't race with rproc_del(), but the existing mechanism can not
be used as it's valid to call rproc_report_crash() in atomic context -
and the "state" is protected by a mutex.

In the event that work is cancelled the pm_stay_awake() is left
unbalanced and need to be unrolled.

The blocking and cancelling of crash-handler work prior to the actual
rproc_shutdown() call does have the explicit side-effect that crashes
resulting from the shutdown process will not enter the crash-handling
path, and as such will not generate devcoredumps etc. Due to the
existing mutual exclusion between these code paths there's no concrete
reduction in functionality, but further work would be needed to handle
this case.