rgw: policy: reuse eval_principal to evaluate the policy principal

rgw: policy: reuse eval_principal to evaluate the policy principal

Since the other edge case when no Principal or a NotPrincipal is supplied also
must be accounted for, which is already done in eval_principal function. Also
reraising the error as Effect::Pass in line with the previous output, though an
Effect::Deny would also work here.

Fixes: https://tracker.ceph.com/issues/46078
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>