selftests: mptcp: close fd_in before returning in main_loop

The file descriptor 'fd_in' is opened when cfg_input is configured, but
not closed in main_loop(), this patch fixes it.

Fixes: 05be5e27 ("selftests: mptcp: add disconnect tests")
Cc: stable@vger.kernel.org
Co-developed-by: Cong Liu <liucong2@kylinos.cn>
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
Signed-off-by: Geliang Tang <tanggeliang@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-v1-3-34161a482a7f@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>