In kernel 4.12 there was introduced new BFQ scheduler and in kernel
5.0 the old CFQ scheduler was removed.  This has an implication on
the cgroups file names.

If the CFQ controller is enabled we use these two files:

    blkio.weight
    blkio.weight_device

The new BFQ controller expose only one file with different name:

    blkio.bfq.weight

The reason is that BFQ controller doesn't support per-device weight.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>