mptcp: pm: only set request_bkup flag when sending MP_PRIO

The 'backup' flag from mptcp_subflow_context structure is supposed to be
set only when the other peer flagged a subflow as backup, not the
opposite.

Fixes: 06706542 ("mptcp: add the outgoing MP_PRIO support")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/pm_netlink.c

@@ -471,7 +471,6 @@ static void __mptcp_pm_send_ack(struct mptcp_sock *msk, struct mptcp_subflow_con
 	slow = lock_sock_fast(ssk);
 	if (prio) {
 		subflow->send_mp_prio = 1;
-		subflow->backup = backup;
 		subflow->request_bkup = backup;
 	}