From Matthew Dobson.

Create and use memblk_online_map.

This patch creates a memblk_online_map, much like cpu_online_map.  It
also creates the standard helper functions, ie: memblk_online(),
num_online_memblks(), memblk_set_online(), memblk_set_offline().

This is used by driverFS topology to keep track of which memory blocks
are in the system and online.