f2fs: add support for FS_IOC_GETFSSYSFSPATH

FS_IOC_GETFSSYSFSPATH ioctl expects sysfs sub-path of a filesystem, the
format can be "$FSTYP/$SYSFS_IDENTIFIER" under /sys/fs, it can helps to
standardizes exporting sysfs datas across filesystems.

This patch wires up FS_IOC_GETFSSYSFSPATH for f2fs, it will output
"f2fs/<dev>".

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