sched: Avoid SMT siblings in select_idle_sibling() if possible

Avoid select_idle_sibling() from picking a sibling thread if there's
an idle core that shares cache.

This fixes SMT balancing in the increasingly common case where there's
a shared cache core available to balance to.

Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1321350377.1421.55.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>