We can requeue tasks for cheaper then doing a complete dequeue followed by
an enqueue.  Add the requeue_task function and perform it where possible.

This will be hit frequently by upcoming changes to the requeueing in
timeslice granularity.
Signed-off-by: Con Kolivas <kernel@kolivas.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>