sched/numa: Fix is_core_idle()

Use the loop variable instead of the function argument to test the
other SMT siblings for idle.

Fixes: ff7db0bf ("sched/numa: Prefer using an idle CPU as a migration target instead of comparing tasks")
Signed-off-by: Mika Penttilä <mika.penttila@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Link: https://lkml.kernel.org/r/20210722063946.28951-1-mika.penttila@gmail.com