mptcp: reset the packet scheduler on PRIO change

Similar to the previous patch, for priority changes
requested by the local PM.
Reported-and-suggested-by: Davide Caratti <dcaratti@redhat.com>
Fixes: 06706542 ("mptcp: add the outgoing MP_PRIO support")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/pm_netlink.c

@@ -727,6 +727,8 @@ static int mptcp_pm_nl_mp_prio_send_ack(struct mptcp_sock *msk,
 		if (!addresses_equal(&local, addr, addr->port))
 			continue;
 
+		if (subflow->backup != bkup)
+			msk->last_snd = NULL;
 		subflow->backup = bkup;
 		subflow->send_mp_prio = 1;
 		subflow->request_bkup = bkup;