selftests/resctrl: Use cache size to determine "fill_buf" buffer size

By default the MBM and MBA tests use the "fill_buf" benchmark to
read from a buffer with the goal to measure the memory bandwidth
generated by this buffer access.

Care should be taken when sizing the buffer used by the "fill_buf"
benchmark. If the buffer is small enough to fit in the cache then
it cannot be expected that the benchmark will generate much memory
bandwidth. For example, on a system with 320MB L3 cache the existing
hardcoded default of 250MB is insufficient.

Use the measured cache size to determine a buffer size that can be
expected to trigger memory access while keeping the existing default
as minimum, now renamed to MINIMUM_SPAN, that has been appropriate for
testing so far.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>