Apply FSFS part of r1584879 to FSX.

* subversion/libsvn_fs_x/lock.c
  (struct unlock_info_t): Add done member.
  (unlock_body): Set done when unlock is successful.
  (svn_fs_fs__lock): Generate an error if there is no lock and no fs_err.
  (svn_fs_fs__unlock): Generate an error if the done flag is not set
   and there is no fs_err.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1584886 13f79535-47bb-0310-9956-ffa450edef68