sched: Fix latencytop and sleep profiling vs group scheduling

The latencytop and sleep accounting code assumes that any
scheduler entity represents a task, this is not so.

Cc: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>