selinux: simplify avc_xperms_audit_required()

By associative and commutative laws, the result of the two 'audited' is
zero. Take the second 'audited' as an example:
  1) audited = requested & avd->auditallow;
  2) audited &= ~requested;
  ==> audited = ~requested & (requested & avd->auditallow);
  ==> audited = (~requested & requested) & avd->auditallow;
  ==> audited = 0 & avd->auditallow;
  ==> audited = 0;

In fact, it is more readable to directly write zero. The value of the
first 'audited' is 0 because AUDIT is not allowed. The second 'audited'
is zero because there is no AUDITALLOW permission.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>