Btrfs: fix possible memory leak in the find_parent_nodes()

In the find_parent_nodes(), if read_tree_block() fails, we can
not return directly, we should free some allocated memory otherwise
memory leak happens.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>