selftests/landlock: Share enforce_ruleset() helper

Move enforce_ruleset() helper function to common.h so that it can be
used both by filesystem tests and network ones.

Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20231026014751.414649-10-konstantin.meskhidze@huawei.com


Signed-off-by: Mickaël Salaün <mic@digikod.net>