sched/fair: Use the busiest group to set prefer_sibling

The prefer_sibling setting acts on the busiest group to move excess tasks
to the local group. This should be done as per request of the child of the
busiest group's sched domain, not the local group's.

Using the flags of the child domain of the local group works fortuitously
if both groups have child domains.

There are cases, however, in which the busiest group's sched domain has
child but the local group's does not. Consider, for instance a non-SMT
core (or an SMT core with only one online sibling) doing load balance with
an SMT core at the MC level. SD_PREFER_SIBLING of the busiest group's child
domain will not be honored. We are left with a fully busy SMT core and an
idle non-SMT core.

Suggested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20230406203148.19182-7-ricardo.neri-calderon@linux.intel.com