Fix compile error

 fs/fs-writeback.c:515:33: error: ‘PAGE_CACHE_SHIFT’ undeclared (first use in this function)
Reported-by: NRandy Dunlap <rdunlap@xenotime.net>
Acked-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>