PM / Hibernate: use rb_entry

Since the software suspend extents are organized in an rbtree, use rb_entry
instead of container_of, as it is semantically more appropriate in order to
get a node as it is iterated.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>