Fix svn_fs_close_root() for FSFS transaction roots.

* subversion/libsvn_fs_fs/tree.c
  (make_txn_root): Make sure the cache lifetime does not exceed
                   the root lifetime.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555491 13f79535-47bb-0310-9956-ffa450edef68