Follow-up to r1589048: Use svn_fs_open2() and scratch pool for svnadmin 
freeze.

* subversion/libsvn_repos/repos.c
  (freeze_baton_t): Add SCRATCH_POOL.
  (multi_freeze): Clear FB->SCRATCH_POOL on entry and use it for temporary 
   allocations.
  (svn_repos_freeze): Initialize FB.SCRATCH_POOL.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1589406 13f79535-47bb-0310-9956-ffa450edef68