selftests: mptcp: join: test both signal & subflow

It should be quite uncommon to set both the subflow and the signal
flags: the initiator of the connection is typically the one creating new
subflows, not the other peer, then no need to announce additional local
addresses, and use it to create subflows.

But some people might be confused about the flags, and set both "just to
be sure at least the right one is set". To verify the previous fix, and
avoid future regressions, this specific case is now validated: the
client announces a new address, and initiates a new subflow from the
same address.

While working on this, another bug has been noticed, where the client
reset the new subflow because an ADD_ADDR echo got received as the 3rd
ACK: this new test also explicitly checks that no RST have been sent by
the client and server.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 86e39e04 ("mptcp: keep track of local endpoint still available for each msk")
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-v1-7-c8a9b036493b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -1989,6 +1989,21 @@ signal_address_tests()
 		chk_add_nr 1 1
 	fi
 
+	# uncommon: subflow and signal flags on the same endpoint
+	# or because the user wrongly picked both, but still expects the client
+	# to create additional subflows
+	if reset "subflow and signal together"; then
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 0 2
+		pm_nl_add_endpoint $ns2 10.0.3.2 flags signal,subflow
+		run_tests $ns1 $ns2 10.0.1.1
+		chk_join_nr 1 1 1
+		chk_add_nr 1 1 0 invert  # only initiated by ns2
+		chk_add_nr 0 0 0         # none initiated by ns1
+		chk_rst_nr 0 0 invert    # no RST sent by the client
+		chk_rst_nr 0 0           # no RST sent by the server
+	fi
+
 	# accept and use add_addr with additional subflows
 	if reset "multiple subflows and signal"; then
 		pm_nl_set_limits $ns1 0 3