blkdeactivate: fix handling of nested mountpoints and mangled mount paths.

blkdeactivate: fix handling of nested mountpoints and mangled mount paths.

If there was a nested mountpoint inside an existing mount path,
blkdeactivate could fail to unmount such a mountpoint as it
needs to deactivate the deepest path first and continue upwards.

For example the simplest reproducer:

[root@rhel6-a ~]# lsblk
NAME                        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda                           8:0    0    4G  0 disk
|-vg-lvol0 (dm-2)           253:2    0   32M  0 lvm  /mnt/a
`-vg-lvol1 (dm-3)           253:3    0   32M  0 lvm  /mnt/a/b

Before this patch:

[root@rhel6-a ~]# blkdeactivate -u
Deactivating block devices:
  UMOUNT: unmounting vg-lvol0 (dm-2) mounted on /mnt/a
umount: /mnt/a: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
  UMOUNT: unmounting vg-lvol1 (dm-3) mounted on /mnt/a/b
  LVM: deactivating Logical Volume vg/lvol1

(deactivation of vg/lvol0 is skipped as /mnt/a that is on lvol0
can't be unmounted - it still has /mnt/a/b as nested mountpoint!)

With this patch applied:

[root@rhel6-a ~]# blkdeactivate -u
Deactivating block devices:
  UMOUNT: unmounting vg-lvol1 (dm-3) mounted on /mnt/a/b
  UMOUNT: unmounting vg-lvol0 (dm-2) mounted on /mnt/a
  LVM: deactivating Logical Volume vg/lvol0
  LVM: deactivating Logical Volume vg/lvol1

===

Also, this patch contains a fix for processing mangled mount paths:

[root@rhel6-a ~]# lsblk
NAME                        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda                           8:0    0    4G  0 disk
`-vg-lvol0 (dm-2)           253:2    0   32M  0 lvm  /mnt/x y z

[root@rhel6-a ~]# lsblk -r
vg-lvol0 253:2 0 32M 0 lvm /mnt/x\x20y\x20z

(the mount path is mangled with \xNN that is visible in raw
lsblk output only and which is used in blkdeactive as well)

Before this patch:

[root@rhel6-a ~]# blkdeactivate -u
Deactivating block devices:
  umount: /mnt/x\x20y\x20z: not found

After this patch applied:

[root@rhel6-a ~]# blkdeactivate -u
Deactivating block devices:
  UMOUNT: unmounting vg-lvol0 (dm-2) mounted on /mnt/x\x20y\x20z
  LVM: deactivating Logical Volume vg/lvol0