f2fs: update i_flags correctly

commit 11f5020d2f43a9a7b9b5cf22873e5a9a06a884f7 upstream.

f2fs enables hash-indexed directory by default, so we need to tag
FS_INDEX_FL in inode::i_flags during directory creataion, in order
to show correct status of inode in lsattr:

Before:
------------------- /mnt/f2fs/dir/
After:
-----------I------- /mnt/f2fs/dir/

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>