f2fs: use F2FS_BLKSIZE to judge bloksize and page_cache_size

In some system PAGE_CACHE_SIZE isn't 4K. So using F2FS_BLKSIZE to judge.

By Jaegeuk Kim:
 o f2fs does not support no other 4KB page cache size.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>