selftests: mptcp: fix incorrect fd checks in main_loop

selftests: mptcp: fix incorrect fd checks in main_loop

Fix a bug where the code was checking the wrong file descriptors
when opening the input files. The code was checking 'fd' instead
of 'fd_in', which could lead to incorrect error handling.

Fixes: 05be5e273c84 ("selftests: mptcp: add disconnect tests")
Cc: stable@vger.kernel.org
Fixes: ca7ae8916043 ("selftests: mptcp: mptfo Initiator/Listener")
Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250328-net-mptcp-misc-fixes-6-15-v1-2-34161a482a7f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>