Add a new flag LANDLOCK_CREATE_RULESET_VERSION to
landlock_create_ruleset(2).  This enables to retreive a Landlock ABI
version that is useful to efficiently follow a best-effort security
approach.  Indeed, it would be a missed opportunity to abort the whole
sandbox building, because some features are unavailable, instead of
protecting users as much as possible with the subset of features
provided by the running kernel.

This new flag enables user space to identify the minimum set of Landlock
features supported by the running kernel without relying on a filesystem
interface (e.g. /proc/version, which might be inaccessible) nor testing
multiple syscall argument combinations (i.e. syscall bisection).  New
Landlock features will be documented and tied to a minimum version
number (greater than 1).  The current version will be incremented for
each new kernel release supporting new Landlock features.  User space
libraries can leverage this information to seamlessly restrict processes
as much as possible while being compatible with newer APIs.

This is a much more lighter approach than the previous
landlock_get_features(2): the complexity is pushed to user space
libraries.  This flag meets similar needs as securityfs versions:
selinux/policyvers, apparmor/features/*/version* and tomoyo/version.

Supporting this flag now will be convenient for backward compatibility.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210422154123.13086-14-mic@digikod.netSigned-off-by: James Morris <jamorris@linux.microsoft.com>