selftests/resctrl: Cleanup bm_pid and ppid usage & limit scope

selftests/resctrl: Cleanup bm_pid and ppid usage & limit scope

'bm_pid' and 'ppid' are global variables. As they are used by different
processes and in signal handler, they cannot be entirely converted into
local variables.

The scope of those variables can still be reduced into resctrl_val.c
only. As PARENT_EXIT() macro is using 'ppid', make it a function in
resctrl_val.c and pass ppid to it as an argument because it is easier
to understand than using the global variable directly.

Pass 'bm_pid' into measure_vals() instead of relying on the global
variable which helps to make the call signatures of measure_vals() and
measure_llc_resctrl() more similar to each other.

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