SUDO: do not imitate full refresh if usn is unknown in smart refresh
    
    USN value should be always known now if at least one full refresh
    was successful.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit c0000a8cc9eccdf5cd8dd72fd6e9bc09d8c7cf00)