f2fs: introduce a new per-sb directory in sysfs

Add a new directory 'stat' in path of /sys/fs/f2fs/<devname>/, later
we can add new readonly stat sysfs file into this directory, it will
make <devname> directory less mess.

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