ceph: fix use-after-free for fsc->mdsc

If the ceph_mdsc_init() fails, it will free the mdsc already.

Reported-by: syzbot+b57f46d8d6ea51960b8c@syzkaller.appspotmail.com
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -4453,7 +4453,6 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 		goto err_mdsc;
 	}
 
-	fsc->mdsc = mdsc;
 	init_completion(&mdsc->safe_umount_waiters);
 	init_waitqueue_head(&mdsc->session_close_wq);
 	INIT_LIST_HEAD(&mdsc->waiting_for_map);
@@ -4508,6 +4507,8 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 
 	strscpy(mdsc->nodename, utsname()->nodename,
 		sizeof(mdsc->nodename));
+
+	fsc->mdsc = mdsc;
 	return 0;
 
 err_mdsmap: