From: Christoph Hellwig <hch@lst.de>

This patch is from the IA64 tree, with minor cleanups from me.

Split out initialization of pgdat->node_mem_map into a separate function
and allow architectures to override it.  This is needed for HP IA64
machines that have a virtually mapped memory map to support big
memory holes without having to use discontigmem.

(memmap_init_zone is non-static to allow the IA64 code to use it -
 I did that instead of passing it's address into the arch hook as
 it is done currently in the IA64 tree)