umount: use devno to identify --all-target devices

 # mount -t tmpfs tmp /mnt/A
 # mount --bind /mnt/A /mnt/B
 # mount -t tmpfs tmp /mnt/C

the command:

  umount --all-targets /mnt/A

has to umount only A and B, but not C. The C is a another filesystem.

Signed-off-by: Karel Zak <kzak@redhat.com>