Do not do StorageInfo() and ListBuckets() for FS/Erasure in health check handler (#7090)

Health checking programs very frequently use /minio/health/live 
to check health, hence we can avoid doing StorageInfo() and 
ListBuckets() for FS/Erasure backend.

cmd/healthcheck-handler.go

@@ -56,18 +56,21 @@ func LivenessCheckHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	s := objLayer.StorageInfo(ctx)
-	// 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)
+	if !globalIsXL && !globalIsDistXL {
+		s := objLayer.StorageInfo(ctx)
+		// Gateways don't provide disk info.
+		if s.Backend.Type == Unknown {
+			// ListBuckets to confirm gateway backend is up
+			if _, err := objLayer.ListBuckets(ctx); err != nil {
+				writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
+				return
+			}
+			writeResponse(w, http.StatusOK, nil, mimeNone)
 			return
 		}
-		writeResponse(w, http.StatusOK, nil, mimeNone)
-		return
 	}
 
+	// For FS and Erasure backend, check if local disks are up.
 	var totalLocalDisks int
 	var erroredDisks int
 	for _, endpoint := range globalEndpoints {