negcache: add fq-usernames of know domains to all UPN neg-caches
The previous patch for this issue did not handle user with
fully-qualified names from known domains correctly. Here the user was
only added to the negative cache of the known domain but not to the
negative UPN caches for all domains. This patch fixes this.
Related to https://pagure.io/SSSD/sssd/issue/3978
Reviewed-by: Jakub Hrozek <firstname.lastname@example.org>