x86_64 can use generic funcs.
For DISCONTIGMEM, CONFIG_OUT_OF_LINE_PFN_TO_PAGE is selected.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>