fix: reduce an extra readdir() attempted on non-legacy setups (#11301)

to verify moving content and preserving legacy content,
we have way to detect the objects through readdir()
this path is not necessary for most common cases on
newer setups, avoid readdir() to save multiple system
calls.

also fix the CheckFile behavior for most common
use case i.e without legacy format.

cmd/xl-storage_test.go

@@ -1719,7 +1719,7 @@ func TestXLStorageCheckFile(t *testing.T) {
 
 	for i, testCase := range testCases {
 		if err := xlStorage.CheckFile(context.Background(), testCase.srcVol, testCase.srcPath); err != testCase.expectedErr {
-			t.Fatalf("TestXLStorage case %d: Expected: \"%s\", got: \"%s\"", i+1, testCase.expectedErr, err)
+			t.Errorf("TestXLStorage case %d: Expected: \"%s\", got: \"%s\"", i+1, testCase.expectedErr, err)
 		}
 	}
 }