umount: check for overlaid mounts

umount: check for overlaid mounts

Linux cannot umount a device whose mount point is hidden by subsequent
mount(s). i.e. We must umount the devices associated to a mount point
in the reverse order of when they were mounted.  If umount was called
violating this rule, report error and exit.

Signed-off-by: Guan Xin <guanx.bac@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>