init-rbdmap: fix for recursive umount

Umount is not always done in the correct order.
For exemple in that case :
/dev/rbd1 on /myrbd
/dev/rbd2 on /myrbd/.snapshots/@GMT-2013.08.09-10.14.44
rbd2 should be umounted before rbd1

Signed-off-by: Laurent Barbe <laurent@ksperis.com>
init-rbdmap: minor fix no rbd exist

Avoid error if stop when no rbd device exist

Signed-off-by: Laurent Barbe <laurent@ksperis.com>