landlock: Refactor merge/inherit_ruleset helpers

Refactor merge_ruleset() and inherit_ruleset() functions to support new
rule types. Add merge_tree() and inherit_tree() helpers.  They use a
specific ruleset's red-black tree according to a key type argument.

Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20231026014751.414649-5-konstantin.meskhidze@huawei.com


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