Validate mountpoint on path-based unmount using statx

Use statx to verify that path-based unmounts proceed only if the
mountpoint reported by statx matches the MNTTAB entry reported by
libzfs, aborting the operation if they differ. Align
zfs umount /path behavior with zfs umount dataset.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17481