rbd: Add support for rbd ROX PVC mounting

if the PVC access mode is ReadOnlyMany
or single node readonly, mounting the rbd
device path to the staging path as readonly
to avoid the write operation.

If the PVC acccess mode is readonly, mapping
rbd images as readonly.

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

internal/rbd/rbd_util.go

@@ -100,6 +100,7 @@ type rbdVolume struct {
 	VolSize             int64  `json:"volSize"`
 	DisableInUseChecks  bool   `json:"disableInUseChecks"`
 	Encrypted           bool
+	readOnly            bool
 	KMS                 util.EncryptionKMS
 
 	// conn is a connection to the Ceph cluster obtained from a ConnPool