mptcp: pm: deny endp with signal + subflow + port

As mentioned in the 'Fixes' commit, the port flag is only supported by
the 'signal' flag, and not by the 'subflow' one. Then if both the
'signal' and 'subflow' flags are set, the problem is the same: the
feature cannot work with the 'subflow' flag.

Technically, if both the 'signal' and 'subflow' flags are set, it will
be possible to create the listening socket, but not to establish a
subflow using this source port. So better to explicitly deny it, not to
create some confusions because the expected behaviour is not possible.

Fixes: 09f12c3a ("mptcp: allow to use port and non-signal in set_flags")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240731-upstream-net-20240731-mptcp-endp-subflow-signal-v1-2-c8a9b036493b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -1328,8 +1328,8 @@ int mptcp_pm_nl_add_addr_doit(struct sk_buff *skb, struct genl_info *info)
 	if (ret < 0)
 		return ret;
 
-	if (addr.addr.port && !(addr.flags & MPTCP_PM_ADDR_FLAG_SIGNAL)) {
-		GENL_SET_ERR_MSG(info, "flags must have signal when using port");
+	if (addr.addr.port && !address_use_port(&addr)) {
+		GENL_SET_ERR_MSG(info, "flags must have signal and not subflow when using port");
 		return -EINVAL;
 	}