PM / sleep: move SNAPSHOT_SET_SWAP_AREA handling into a helper

Move the handling of the SNAPSHOT_SET_SWAP_AREA ioctl from the main
ioctl helper into a helper function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>