rbd: fix cleanup when trying to mount inexistent image
Previously we didn't clean up the sysfs entry that was just
created.
Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>
authored by Yehuda Sadeh and committed by Sage Weil 766fc439 f363e45f