selftests: mptcp: join: check removing signal+subflow endp

This validates the previous commit: endpoints with both the signal and
subflow flags should always be marked as used even if it was not
possible to create new subflows due to the MPTCP PM limits.

For this test, an extra endpoint is created with both the signal and the
subflow flags, and limits are set not to create extra subflows. In this
case, an ADD_ADDR is sent, but no subflows are created. Still, the local
endpoint is marked as used, and no warning is fired when removing the
endpoint, after having sent a RM_ADDR.

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: 85df533 ("mptcp: pm: do not ignore 'subflow' if 'signal' flag is also set")
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/20260303-net-mptcp-misc-fixes-7-0-rc2-v1-5-4b5462b6f016@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

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

@@ -2626,6 +2626,19 @@ remove_tests()
 		chk_rst_nr 0 0
 	fi
 
+	# signal+subflow with limits, remove
+	if reset "remove signal+subflow with limits"; then
+		pm_nl_set_limits $ns1 0 0
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal,subflow
+		pm_nl_set_limits $ns2 0 0
+		addr_nr_ns1=-1 speed=slow \
+			run_tests $ns1 $ns2 10.0.1.1
+		chk_join_nr 0 0 0
+		chk_add_nr 1 1
+		chk_rm_nr 1 0 invert
+		chk_rst_nr 0 0
+	fi
+
 	# addresses remove
 	if reset "remove addresses"; then
 		pm_nl_set_limits $ns1 3 3