Min_low_pfn and max_low_pfn were used in pfn_valid macro if defined
CONFIG_FLATMEM.  When the functions that use the pfn_valid is used in
driver module, max_low_pfn and min_low_pfn is to undefined, and fail to
build.

  ERROR: "min_low_pfn" [drivers/block/aoe/aoe.ko] undefined!
  ERROR: "max_low_pfn" [drivers/block/aoe/aoe.ko] undefined!
  make[2]: *** [__modpost] Error 1
  make[1]: *** [modules] Error 2

This patch fix this problem.
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>