init-rbdmap: fix error on stop rbdmap

Avoid an error on stop service if many /dev/rbd* exist.

Signed-off-by: Laurent Barbe <laurent@ksperis.com>
Merge pull request #525 from ksperis/rbdmap.init-fix

init-rbdmap: fix error on stop rbdmap

Reviewed-by: Sage Weil <sage@inktank.com>