Need 'EXPORT_SYMBOL(clear_page)' if building with allmodconfig.

The related errors:
  ERROR: "clear_page" [fs/ocfs2/dlm/ocfs2_dlm.ko] undefined!
  ERROR: "clear_page" [fs/ntfs/ntfs.ko] undefined!
  ERROR: "clear_page" [fs/gfs2/gfs2.ko] undefined!
  ERROR: "clear_page" [fs/fuse/fuse.ko] undefined!
  ERROR: "clear_page" [fs/ext3/ext3.ko] undefined!
  ERROR: "clear_page" [fs/ext2/ext2.ko] undefined!
  ERROR: "clear_page" [fs/exofs/libore.ko] undefined!
  ERROR: "clear_page" [fs/exofs/exofs.ko] undefined!
  ERROR: "clear_page" [drivers/block/brd.ko] undefined!
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>