cache_req: use own namespace for UPNs
    
    If the UPN use the same domain name as the configured domain an
    unsuccessful lookup by name will already create an entry in the negative
    cache. If the lookup by UPN would use the same namespace the lookup will
    immediately be finished because there would already be an entry in the
    negative cache.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3313
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>