sched: Restrict nohz balance kicks to stay in the HMP domain

sched: Restrict nohz balance kicks to stay in the HMP domain

There is little point in doing a nohz balance kick on a CPU from a
different HMP domain, since the unset SD_LOAD_BALANCE flag on the CPU
domain level prevents tasks from being balanced across clusters
except through the per-task load driven hmp_migrate/hmp_offload paths.

Further, the nohz balance kick is actively harmful to power usage if
all the tasks fit into the little domain since it causes the big
domain to wake up and do a lot of calculation to determine that
there is nothing to do.

A more generic solution is to walk the sched domain tree and determine
the intersection of potential idle balance cpus with visibility of
tasks on the current CPU, however HMP domains are more easily
accessible.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
[k.kozlowski: rebased on 4.1, no signed-off-by of previous committer]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>