PM: hibernate: add sysfs interface for hibernate_compression_threads

PM: hibernate: add sysfs interface for hibernate_compression_threads

Add a sysfs attribute `/sys/power/hibernate_compression_threads` to
allow runtime configuration of the number of threads used for
compressing and decompressing hibernation images.

The new sysfs interface enables dynamic adjustment at runtime:

    # cat /sys/power/hibernate_compression_threads
    3
    # echo 4 > /sys/power/hibernate_compression_threads

This change provides greater flexibility for debugging and performance
tuning of hibernation without requiring a reboot.

Signed-off-by: Xueqin Luo <luoxueqin@kylinos.cn>
Link: https://patch.msgid.link/c68c62f97fabf32507b8794ad8c16cd22ee656ac.1761046167.git.luoxueqin@kylinos.cn
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>