[PATCH] pgdat allocation and update for ia64 of memory hotplug: hold pgdat address at system running

This is a preparatory patch to make common code for updating of NODE_DATA() of
ia64 between boottime and hotplug.

Current code remembers pgdat address in mem_data which is used at just boot
time.  But its information can be used at hotplug time by moving to global
value.  The next patch uses this array.
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>