From: jbarnes@sgi.com (Jesse Barnes)

Currently, free_all_bootmem_core() assumes that the bdata for a given
node will begin where the node's memory map begins.  This isn't
necessarily true on machines that use a virtual memory map (e.g. ia64
discontig machines), so we fix page to point to the first actual page of
RAM on the node, which _does_ contain the bdata struct.