selftests/mm: gup_longterm: fix a resource leak

The opened file should be closed in run_with_tmpfile(), otherwise resource
leak will occur

Link: https://lkml.kernel.org/r/20230831093144.7520-1-dingxiang@cmss.chinamobile.com


Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>