PM: sleep: Add locking to dpm_async_resume_children()

Commit 0cbef962ce1f ("PM: sleep: Resume children after resuming the
parent") introduced a subtle concurrency issue that may lead to a kernel
crash if system suspend is aborted and may also slow down asynchronous
device resume otherwise.

Namely, the initial list walks in dpm_noirq_resume_devices(),
dpm_resume_early(), and dpm_resume() call dpm_clear_async_state() for
every device and attempt to asynchronously resume it if it has no
children (so it is a "root" device).  The asynchronous resume of a
root device triggers an attempt to asynchronously resume its children
which may take place before calling dpm_clear_async_state() for them
due to the lack of synchronization between dpm_async_resume_children()
and the code calling dpm_clear_async_state().  If this happens, the
dpm_clear_async_state() that comes in late, will clear
power.work_in_progress for the given device after it has been set by
__dpm_async(), so the suspend callback will be allowed to run once
again for the same device during the same transition.  This leads to
a whole range of interesting breakage.

Fortunately, if the suspend transition is not aborted, power.work_in_progress
is set by it for all devices, so dpm_async_resume_children() will not
schedule asynchronous resume for them until dpm_clear_async_state()
clears that flag, but this means missing an opportunity to start the
resume of those devices earlier.

Address the above issue by adding dpm_list_mtx locking to
dpm_async_resume_children(), so it will wait for the entire initial
list walk and the invocation of dpm_clear_async_state() for all devices
to be completed before scheduling any new asynchronous resume callbacks.

Fixes: 0cbef962ce1f ("PM: sleep: Resume children after resuming the parent")
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/4280
Reported-and-tested-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/13779172.uLZWGnKmhe@rjwysocki.net

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