selftests/resctrl: Refactor fill_buf functions

There are unnecessary nested calls in fill_buf.c:
  - run_fill_buf() calls fill_cache()
  - alloc_buffer() calls malloc_and_init_memory()

Simplify the code flow and remove those unnecessary call levels by
moving the called code inside the calling function and remove the
duplicated error print.

Resolve the difference in run_fill_buf() and fill_cache() parameter
name into 'buf_size' which is more descriptive than 'span'. Also, while
moving the allocation related code, rename 'p' into 'buf' to be
consistent in naming the variables.

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