[PATCH] define for_each_online_pgdat

This patch defines for_each_online_pgdat() as a replacement of
for_each_pgdat()

Now, online nodes are managed by node_online_map.  But for_each_pgdat()
uses pgdat_link to iterate over all nodes(pgdat).  This means management
structure for online pgdat is duplicated.

I think using node_online_map for for_each_pgdat() is simple and sane
rather ather than pgdat_link.  New macro is named as
for_each_online_pgdat().  Following patch will fix callers of
for_each_pgdat().

The bootmem allocater uses for_each_pgdat() before pgdat initialization.  I
don't think it's sane.  Following patch will fix it.

Signed-off-by: Yasunori Goto     <y-goto@jp.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>