selftests/resctrl: Remove unnecessary startptr global from fill_buf

selftests/resctrl: Remove unnecessary startptr global from fill_buf

fill_buf stores buffer pointer into global variable startptr that is
only used in fill_cache().

Remove startptr as global variable, the local variable in fill_cache()
is enough to keep the pointer.

Signed-off-by: Ilpo Järvinen <[email protected]>
Reviewed-by: Reinette Chatre <[email protected]>
Tested-by: Babu Moger <[email protected]>
Tested-by: Shaopeng Tan (Fujitsu) <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>