config: disable kfence by default (bsc#1185565)
    
    Apperently the overhead of kfence is not as negligible as the help text
    seemed to promise so that it seems more appropriate to disable kfence by
    default by setting CONFIG_KFENCE_SAMPLE_INTERVAL to 0. Anyone who wants to
    enable it can still do so using the kfence.sample_interval command line
    parameter.