audit: use audit_set_enabled() in audit_enable()

Use audit_set_enabled() to enable auditing during early boot.  This
obviously won't emit an audit change record, but it will work anyway
and should help prevent in future problems by consolidating the
enable/disable code in one function.

Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>