Currently, fsfreeze-freeze may cause deadlock if a guest has loopback mounts
of image files in its disk; e.g.:

    # mount | grep ^/
    /dev/vda1 / type ext4 (rw,noatime,seclabel,data=ordered)
    /tmp/disk.img on /mnt type ext4 (rw,relatime,seclabel)

To avoid the deadlock, this freezes filesystems in reverse order of mounts.
Signed-off-by: NTomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
*fix up commit msg
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>