samples/bpf: Fix a resource leak

The opened file should be closed in show_sockopts(), otherwise resource
leak will occur that this problem was discovered by reading code

Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241010014126.2573-1-zhujun2@cmss.chinamobile.com