checkpatch: default to success if only warnings

CHK-level checks have been removed from checkpatch or bumped to
errors, so there is no effect anymore for --strict/--subjective.
Furthermore, even most WARNs have been bumped to errors, with
WARN only reserved to things that patchew probably ought not
to complain about (and that maintainers probably will notice
anyway during review if they are extreme).

Default to exiting with success even if there are WARN-level
failures, and cause --strict to fail for warnings.  Maintainers
that want to have a strict 80-character limit for their subsystem
can add it to a commit hook for example.

The --subjective synonym is removed.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
