selftests: mptcp: join: validate fullmesh endp on 1st sf

This case was not covered, and the wrong ID was set before the previous
commit.

The rest is not modified, it is just that it will increase the code
coverage.

The right address ID can be verified by looking at the packet traces. We
could automate that using Netfilter with some cBPF code for example, but
that's always a bit cryptic. Packetdrill seems better fitted for that.

Fixes: 4f49d633 ("selftests: mptcp: add fullmesh testcases")
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/20240819-net-mptcp-pm-reusing-id-v1-13-38035d40de5b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -3059,6 +3059,7 @@ fullmesh_tests()
 		pm_nl_set_limits $ns1 1 3
 		pm_nl_set_limits $ns2 1 3
 		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
+		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,fullmesh
 		fullmesh=1 speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
 		chk_join_nr 3 3 3