nss: use enumeration context as talloc parent for cache req result
    
    Otherwise we end up with memory leak since the result is never freed.
    
    We need to convert nctx->*ent structures into talloc pointer so
    we can use enum_ctx as parent.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3870
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>