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>