fs: Fix dir open during failure condition

Should leave the Dir in an openable state, currently does not
since it thinks it's in use by the fs it failed to open on.

features/filesystem/Dir.cpp

@@ -43,8 +43,12 @@ int Dir::open(FileSystem *fs, const char *path)
         return -EINVAL;
     }
 
-    _fs = fs;
-    return _fs->dir_open(&_dir, path);
+    int err = fs->dir_open(&_dir, path);
+    if (!err) {
+        _fs = fs;
+    }
+
+    return err;
 }
 
 int Dir::close()