Follow-up to r1358516: Use proper pool for config in FSFS svn_fs_t object.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__open): Use fs->pool for call to read_config().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1589097 13f79535-47bb-0310-9956-ffa450edef68