Fix healthcheck for NAS gateway (#6452)

It was expected that in gateway mode, we do not know
the backend types whereas in NAS gateway since its
an extension of FS mode (standalone) this leads to
an issue in LivenessCheckHandler() which would perpetually
return 503, this would affect all kubernetes, openshift
deployments of NAS gateway.

cmd/healthcheck-handler.go

@@ -57,8 +57,8 @@ func LivenessCheckHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	s := objLayer.StorageInfo(ctx)
-	// Gateways don't provide disk info
-	if s.Backend.Type == Unknown {
+	// Gateways don't provide disk info, also handle special case for NAS gateway.
+	if s.Backend.Type == Unknown || s.Backend.Type == BackendFS {
 		// ListBuckets to confirm gateway backend is up
 		if _, err := objLayer.ListBuckets(ctx); err != nil {
 			writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
@@ -84,6 +84,7 @@ func LivenessCheckHandler(w http.ResponseWriter, r *http.Request) {
 			}
 		}
 	}
+
 	// If all exported local disks have errored, we simply let kubernetes
 	// take us down.
 	if totalLocalDisks == erroredDisks {