PM / Hibernate: Implement position keeping in radix tree

Add code to remember the last position that was requested in
the radix tree. Use it as a cache for faster linear walking
of the bitmap in the memory_bm_rtree_next_pfn() function
which is also added with this patch.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>