rbd: move flattening to helper function
rbd: move flattening to helper function

in NodeStage operation we are flattening
the image to support mounting on the older
clients. this commits moves it to a helper
function to reduce code complexity.

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

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