Fix s4u2self with adtrust
    
    When ADtrust is installed we add a PAC to all tickets, during protocol
    transition we need to generate a new PAC for the requested user ticket,
    not check the existing PAC on the requestor ticket.
    
    https://pagure.io/freeipa/issue/6862
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>