mufs: allocate a block for the root, if no root has been allocated in mufs_alloc_page()

source/fs/mufs/tree.c

@@ -49,6 +49,9 @@ int mufs_lookup_page(struct mufs *fs, struct mufs_tree *tree, uint32_t *page,
 int mufs_alloc_page(struct mufs *fs, struct mufs_tree *tree, uint32_t *page,
 	uint32_t va)
 {
+	if (!tree->root && mufs_alloc_block(fs, &tree->root) < 0)
+		return -1;
+
 	return mufs_do_lookup(fs, page, tree->root, tree->depth, va, 1);
 }