selftests: mptcp: always close input's FD if opened

commit 7c70bcc2 upstream.

In main_loop_s function, when the open(cfg_input, O_RDONLY) function is
run, the last fd is not closed if the "--cfg_repeat > 0" branch is not
taken.

Fixes: 05be5e27

 ("selftests: mptcp: add disconnect tests")
Cc: stable@vger.kernel.org
Signed-off-by: Liu Jing <liujing@cmss.chinamobile.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>