selftests: mptcp: join: rm: set backup flag

selftests: mptcp: join: rm: set backup flag

Some of these 'remove' tests rarely fail because a subflow has been
reset instead of cleanly removed. This can happen when one extra subflow
which has never carried data is being closed (FIN) on one side, while
the other is sending data for the first time.

To avoid such subflows to be used right at the end, the backup flag has
been added. With that, data will be only carried on the initial subflow.

Fixes: d2c4333a801c ("selftests: mptcp: add testcases for removing addrs")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251110-net-mptcp-sft-join-unstable-v1-2-a4332c714e10@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>