Avoid ListBuckets() call instead rely on simple HTTP GET (#8475)

This is to avoid making calls to backend and requiring
gateways to allow permissions for ListBuckets() operation
just for Liveness checks, we can avoid this and make
our liveness checks to be more performant.

cmd/gateway/b2/gateway-b2.go

@@ -123,7 +123,10 @@ func (g *B2) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, error)
 	return &b2Objects{
 		creds:    creds,
 		b2Client: client,
-		ctx:      ctx,
+		httpClient: &http.Client{
+			Transport: minio.NewCustomHTTPTransport(),
+		},
+		ctx: ctx,
 	}, nil
 }
 
@@ -135,10 +138,11 @@ func (g *B2) Production() bool {
 // b2Object implements gateway for MinIO and BackBlaze B2 compatible object storage servers.
 type b2Objects struct {
 	minio.GatewayUnsupported
-	mu       sync.Mutex
-	creds    auth.Credentials
-	b2Client *b2.B2
-	ctx      context.Context
+	mu         sync.Mutex
+	creds      auth.Credentials
+	b2Client   *b2.B2
+	httpClient *http.Client
+	ctx        context.Context
 }
 
 // Convert B2 errors to minio object layer errors.
@@ -225,12 +229,13 @@ func b2MsgCodeToObjectError(code int, msgCode string, msg string, params ...stri
 // Shutdown saves any gateway metadata to disk
 // if necessary and reload upon next restart.
 func (l *b2Objects) Shutdown(ctx context.Context) error {
-	// TODO
 	return nil
 }
 
 // StorageInfo is not relevant to B2 backend.
 func (l *b2Objects) StorageInfo(ctx context.Context) (si minio.StorageInfo) {
+	si.Backend.Type = minio.BackendGateway
+	si.Backend.GatewayOnline = minio.IsBackendOnline(ctx, l.httpClient, "https://api.backblazeb2.com/b2api/v1")
 	return si
 }