sched: Clean up rebalance_domains() load-balance interval calculation

Instead of the possible multiple-evaluation of num_online_cpus()
in rebalance_domains() that Linus reported, avoid it altogether
in the normal case since it's implemented with a Hamming weight
function over a cpu bitmask which can be darn expensive for those
with big iron.

This also makes it cleaner, smaller and documents the code.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1301991265.2225.12.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -6331,6 +6331,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		break;
 #endif
 	}
+
+	update_max_interval();
+
 	return NOTIFY_OK;
 }