ceph: avoid invalid memory dereference in the middle of umount

extra_mon_dispatch() and debugfs' foo_show functions dereference
fsc->mdsc. we should clean up fsc->client->extra_mon_dispatch
and debugfs before destroying fsc->mds.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>