selftests/landlock: Fix a resource leak

The opened file should be closed before return, otherwise resource leak
will occur.

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20230830101148.3738-1-dingxiang@cmss.chinamobile.com
Fixes: 3de64b65

 ("selftests/landlock: Add supports_filesystem() helper")
Signed-off-by: Mickaël Salaün <mic@digikod.net>