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: 05be5e2 ("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>

tools/testing/selftests/net/mptcp/mptcp_connect.c

@@ -1299,7 +1299,7 @@ int main_loop(void)
 
 	ret = copyfd_io(fd_in, fd, 1, 0, &winfo);
 	if (ret)
-		return ret;
+		goto out;
 
 	if (cfg_truncate > 0) {
 		shutdown(fd, SHUT_WR);
@@ -1320,7 +1320,10 @@ int main_loop(void)
 		close(fd);
 	}
 
-	return 0;
+out:
+	if (cfg_input)
+		close(fd_in);
+	return ret;
 }
 
 int parse_proto(const char *proto)