zonefs: 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 zonefs, it will output
"zonefs/<dev>".

Signed-off-by: Liao Chen <liaochen4@huawei.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>