landlock: Add ruleset and domain management

A Landlock ruleset is mainly a red-black tree with Landlock rules as
nodes.  This enables quick update and lookup to match a requested
access, e.g. to a file.  A ruleset is usable through a dedicated file
descriptor (cf. following commit implementing syscalls) which enables a
process to create and populate a ruleset with new rules.

A domain is a ruleset tied to a set of processes.  This group of rules
defines the security policy enforced on these processes and their future
children.  A domain can transition to a new domain which is the
intersection of all its constraints and those of a ruleset provided by
the current process.  This modification only impact the current process.
This means that a process can only gain more constraints (i.e. lose
accesses) over time.

Cc: James Morris <[email protected]>
Signed-off-by: Mickaël Salaün <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Jann Horn <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: James Morris <[email protected]>

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