PM: hibernate: move finding the resume device out of software_resume

PM: hibernate: move finding the resume device out of software_resume

software_resume can be called either from an init call in the boot code,
or from sysfs once the system has finished booting, and the two
invocation methods this can't race with each other.

For the latter case we did just parse the suspend device manually, while
the former might not have one.  Split software_resume so that the search
only happens for the boot case, which also means the special lockdep
nesting annotation can go away as the system transition mutex can be
taken a little later and doesn't have the sysfs locking nest inside it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230531125535.676098-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>