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>

fs/ceph/mds_client.c

@@ -3769,13 +3769,13 @@ static void ceph_mdsc_stop(struct ceph_mds_client *mdsc)
 void ceph_mdsc_destroy(struct ceph_fs_client *fsc)
 {
 	struct ceph_mds_client *mdsc = fsc->mdsc;
-
 	dout("mdsc_destroy %p\n", mdsc);
-	ceph_mdsc_stop(mdsc);
 
 	/* flush out any connection work with references to us */
 	ceph_msgr_flush();
 
+	ceph_mdsc_stop(mdsc);
+
 	fsc->mdsc = NULL;
 	kfree(mdsc);
 	dout("mdsc_destroy %p done\n", mdsc);

fs/ceph/super.c

@@ -636,8 +636,6 @@ static void destroy_fs_client(struct ceph_fs_client *fsc)
 
 	destroy_mount_options(fsc->mount_options);
 
-	ceph_fs_debugfs_cleanup(fsc);
-
 	ceph_destroy_client(fsc->client);
 
 	kfree(fsc);
@@ -1040,6 +1038,10 @@ static void ceph_kill_sb(struct super_block *s)
 
 	ceph_mdsc_pre_umount(fsc->mdsc);
 	generic_shutdown_super(s);
+
+	fsc->client->extra_mon_dispatch = NULL;
+	ceph_fs_debugfs_cleanup(fsc);
+
 	ceph_mdsc_destroy(fsc);
 
 	destroy_fs_client(fsc);