Validate mountpoint on path-based unmount using statx

[ixhamza]

Motivation and Context

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.

Supersedes: #17421

Description

When zfs umount /path is run on a bind mount, it silently unmounts the underlying dataset. This patch uses statx to detect bind mounts accurately and validate the specified path against the MNTTAB entry reported by libzfs before performing the unmount.

How Has This Been Tested?

Before

buildroot@root:~# zpool create -O mountpoint=/mnt/tank tank nvme0n1
buildroot@root:~# mkdir -p /mnt/tank-bind
buildroot@root:~# mount --bind /mnt/tank /mnt/tank-bind
buildroot@root:~# zfs umount /mnt/tank-bind
buildroot@root:~# mount | grep tank
tank on /mnt/tank-bind type zfs (rw,relatime,xattr,noacl,casesensitive)

After

buildroot@root:~# zpool create -O mountpoint=/mnt/tank tank nvme0n1
buildroot@root:~# mkdir -p /mnt/tank-bind
buildroot@root:~# mount --bind /mnt/tank /mnt/tank-bind
buildroot@root:~# zfs umount /mnt/tank-bind
cannot unmount '/mnt/tank-bind': not a original mountpoint
buildroot@root:~# mount | grep tank
tank on /mnt/tank type zfs (rw,relatime,xattr,noacl,casesensitive)
tank on /mnt/tank-bind type zfs (rw,relatime,xattr,noacl,casesensitive)
buildroot@root:~# umount /mnt/tank-bind
buildroot@root:~# mount | grep tank
tank on /mnt/tank type zfs (rw,relatime,xattr,noacl,casesensitive)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.