sched: pull only one task during NEWIDLE balancing to limit critical section

sched: pull only one task during NEWIDLE balancing to limit critical section

git-id c4acb2c0669c5c5c9b28e9d02a34b5c67edf7092 attempted to limit
newidle critical section length by stopping after at least one task
was moved.  Further investigation has shown that there are other
paths nested further inside the algorithm which still remain that allow
long latencies to occur with newidle balancing.  This patch applies
the same technique inside balance_tasks() to limit the duration of
this optional balancing operation.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
CC: Nick Piggin <npiggin@suse.de>