mds: Fix use of auto prototype, it is a non std extension

mds: Fix use of auto prototype, it is a non std extension

Clang complains:
In file included from /home/jenkins/workspace/ceph-master/src/msg/Message.cc:128:
In file included from /home/jenkins/workspace/ceph-master/src/messages/MFSMap.h:20:
/home/jenkins/workspace/ceph-master/src/mds/FSMap.h:330:49: error: 'auto' not allowed in function prototype
  void modify_filesystem(fs_cluster_id_t fscid, auto&& fn)
                                                ^~~~
/home/jenkins/workspace/ceph-master/src/mds/FSMap.h:341:37: error: 'auto' not allowed in function prototype
  void modify_daemon(mds_gid_t who, auto&& fn)
                                    ^~~~

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>