PM / sleep: Refactor filesystems sync to reduce duplication

Create a common helper to sync filesystems for system suspend and
hibernation.

Signed-off-by: Harry Pan <harry.pan@intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>