Add an above_background_load() function which can be used by other
subsystems to detect if there is anything besides niced tasks running.

Place it in sched.h to allow it to be compiled out if not used.

Unused for now, but it is a useful hint to the IO scheduler and to
swap-prefetch.
Signed-off-by: Con Kolivas <kernel@kolivas.org>
Cc: Peter Williams <pwil3058@bigpond.net.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>