kcm: Call strp_stop before strp_done in kcm_attach

In kcm_attach strp_done is called when sk_user_data is already
set to fail the attach. strp_done needs the strp to be stopped and
warns if it isn't. Call strp_stop in this case to eliminate the
warning message.

Reported-by: syzbot+88dfb55e4c8b770d86e3@syzkaller.appspotmail.com
Fixes: e5571240 ("kcm: Check if sk_user_data already set in kcm_attach"
Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -1417,6 +1417,7 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 	 */
 	if (csk->sk_user_data) {
 		write_unlock_bh(&csk->sk_callback_lock);
+		strp_stop(&psock->strp);
 		strp_done(&psock->strp);
 		kmem_cache_free(kcm_psockp, psock);
 		return -EALREADY;