ANDROID: sched: Add a reserved field for vendors in sched_avg

One such usage for the reserved member is to allow for more accurate
frequency responses for vCPU threads in virtualization usecases. See
link below for more context.

Having the reserved member as part of sched_avg struct instead of
task_struct or sched_entity allows it to be accessable within the same
cacheline as the other PELT signals, which results in better
performance.

Bug: 284368385
Link: https://lore.kernel.org/all/20230330224348.1006691-2-davidai@google.com/


Change-Id: I0f3b90bcbbf98017b2227303c481c76ffaa1139f
Signed-off-by: David Dai <davidai@google.com>