sched/topology: Check SDF_SHARED_CHILD in highest_flag_domain()

Do not assume that all the children of a scheduling domain have a given
flag. Check whether it has the SDF_SHARED_CHILD meta flag.

Suggested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230406203148.19182-9-ricardo.neri-calderon@linux.intel.com