selftests/resctrl: Improve parameter consistency in fill_buf

fill_buf's arguments can be improved in multiple ways:

  - Multiple functions in fill_buf have start_ptr as one of their
    argument which is a bit long and the extra "start" is pretty
    obvious when it comes to pointers.

  - Some of the functions take end_ptr and others size_t to indicate
    the end of the buffer.

  - Some arguments meaning buffer size are called just 's'

  - mem_flush() takes void * but immediately converts it to char *

Cleanup the parameters to make things simpler and more consistent:

  - Rename start_ptr to simply buf as it's shorter.

  - Replace end_ptr and s parameters with buf_size and only calculate
    end_ptr in the functions that truly use it.

  - Make mem_flush() parameters to follow the same convention as the
    other functions in fill_buf.

  - convert mem_flush() char * to unsigned char *.

While at it, fix also a typo in a comment.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>