rbd: consider rbd as default mounter if not set.

For the default mounter the mounter option
will not be set in the storageclass and as it is
not available in the storageclass same will not
be set in the volume context, Because of this the
mapOptions are getting discarded. If the mounter
is not set assuming it's an rbd mounter.

Note:- If the mounter is not set in the storageclass
we can set it in the volume context explicitly,
Doing this check-in node server to support backward
existing volumes and the check is minimal we are not
altering the volume context.

fixes: #3076

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

internal/rbd/nodeserver.go

@@ -241,7 +241,11 @@ func populateRbdVol(
 		return nil, status.Error(codes.Internal, err.Error())
 	}
 
-	if req.GetVolumeContext()["mounter"] == rbdDefaultMounter &&
+	mounter := req.GetVolumeContext()["mounter"]
+	if mounter == "" {
+		mounter = rbdDefaultMounter
+	}
+	if mounter == rbdDefaultMounter &&
 		!isKrbdFeatureSupported(ctx, strings.Join(rv.ImageFeatureSet.Names(), ",")) {
 		if !parseBoolOption(ctx, req.GetVolumeContext(), tryOtherMounters, false) {
 			log.ErrorLog(ctx, "unsupported krbd Feature, set `tryOtherMounters:true` or fix krbd driver")
@@ -252,7 +256,7 @@ func populateRbdVol(
 		// fallback to rbd-nbd,
 		rv.Mounter = rbdNbdMounter
 	} else {
-		rv.Mounter = req.GetVolumeContext()["mounter"]
+		rv.Mounter = mounter
 	}
 
 	err = getMapOptions(req, rv)