nilfs2: add support for FS_IOC_GETFSSYSFSPATH

Use the standard helper super_set_sysfs_name_bdev() to give the sysfs
subpath of the filesystem for the FS_IOC_GETFSSYSFSPATH ioctl.

For nilfs2, it will output "nilfs2/<dev>".

Link: https://lkml.kernel.org/r/20240815074408.5550-3-konishi.ryusuke@gmail.com


Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>