PM: sleep: Add support for wakeup during filesystem sync

Add helper function pm_sleep_fs_sync() and related data structures
as a preparation for allowing system suspend and hibernation to be
aborted by wakeup events while syncing file systems.

The new function, to be called by the suspend process in order to
sync file systems, uses a dedicated ordered workqueue to run
ksys_sync_helper() in parallel with the calling process.  Next, it
waits for the completion of the filesystem sync and periodically
checks if any system wakeup events are pending, in which case it will
return an error.

If that happens while the filesystem sync is still in progress, it
will continue, possibly after pm_sleep_fs_sync() has returned, and if
that function is called again before the sync is complete, a new work
item to run ksys_sync_helper() again will be queued (and waited for)
to increase the likelihood of writing all of the dirty pages in memory
back to persistent storage.

Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Samuel Wu <wusamuel@google.com>
Co-developed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
[ rjw: Subject and changelog rewrite, tags adjustment ]
Link: https://patch.msgid.link/20251119171426.4086783-2-wusamuel@google.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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