back-sch: reuse backend_should_descend
    
    When backend_search_find_set_dn_cb() is called, use the same logic as in
    other callbacks -- identify whether we should descend into the group by
    using backend_should_descend().
    
    The issue was introduced in 2015 with ID Views support but was masked
    until 61ea8f6a104da25329e301a8f56944f860de8177 as we always felt through
    to the full scan of the groups anyway. with the latter change the
    fell-through part was removed.
    
    Resolves: rhbz#1958909
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Signed-off-by: Thierry Bordaz <tbordaz@redhat.com>