f2fs: convert to F2FS_*_INO macro

Use F2FS_ROOT_INO, F2FS_NODE_INO and F2FS_META_INO macro
for better code readability.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Shaohua Liu <liush@allwinnertech.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>