parentDirIsObject() to return quickly with inexistant parent (#11204)

Rewrite parentIsObject() function. Currently if a client uploads
a/b/c/d, we always check if c, b, a are actual objects or not.

The new code will check with the reverse order and quickly quit if 
the segment doesn't exist.

So if a, b, c in 'a/b/c' does not exist in the first place, then returns
false quickly.

cmd/xl-storage.go

@@ -1757,10 +1757,10 @@ func (s *xlStorage) CheckFile(ctx context.Context, volume string, path string) e
 	}
 
 	// Stat a volume entry.
-	_, err = os.Stat(volumeDir)
+	_, err = os.Stat(pathJoin(volumeDir, path))
 	if err != nil {
 		if osIsNotExist(err) {
-			return errVolumeNotFound
+			return errPathNotFound
 		}
 		return err
 	}