rbd: unmap rbd image if the mounting fails
rbd: unmap rbd image if the mounting fails

There is a bug in current code where the devicePath
is always empty and the rbd image unmap never
happens if nodeplugin fails to mount the rbd image
to the stagingpath.
This is a fix to unmap the rbd image if some issue
occurs after rbd image is mapped.

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi