PAM: Fix domain for UPN based lookups
    
    Since sysdb_search_user_by_upn() searches the whole cache we have to set
    the domain so that it matches the result.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 9b8fcf685c5ca70a5067a621385bcdc8d9fd6469)