sched/numa: Stop an exhastive search if a reasonable swap candidate or idle CPU is found

When domains are imbalanced or overloaded a search of all CPUs on the
target domain is searched and compared with task_numa_compare. In some
circumstances, a candidate is found that is an obvious win.

o A task can move to an idle CPU and an idle CPU is found
o A swap candidate is found that would move to its preferred domain

This patch terminates the search when either condition is met.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>