selftests: mptcp: connect cleanup TFO setup

selftests: mptcp: connect cleanup TFO setup

To the TFO, only the file descriptor is needed, the family is not.

Also, the error can be handled the same way when 'sendto()' or
'connect()' are used. Only the printed error message is different.

This avoids a bit of confusions.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260203-net-next-mptcp-misc-feat-6-20-v1-14-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>