[IA64] fix the difference between node_mem_map and node_start_pfn

makedumpfile[1] cannot run on ia64 discontigmem kernel, because the member
node_mem_map of struct pgdat_list has invalid value.  This patch fixes it.

node_start_pfn shows the start pfn of each node, and node_mem_map should
point 'struct page' of each node's node_start_pfn.  On my machine, node0's
node_start_pfn shows 0x400 and its node_mem_map points 0xa0007fffbf000000.
 This address is the same as vmem_map, so the node_mem_map points 'struct
page' of pfn 0, even if its node_start_pfn shows 0x400.

The cause is due to the round down of min_pfn in count_node_pages() and
node0's node_mem_map points 'struct page' of inactive pfn (0x0).  This
patch fixes it.

makedumpfile[1]: dump filtering command
https://sourceforge.net/projects/makedumpfile/

Signed-off-by: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Cc: Bernhard Walle <bwalle@suse.de>
Cc: Jay Lan <jlan@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>