numa: add a sysctl for numa_balancing

Add a working sysctl to enable/disable automatic numa memory balancing
at runtime.

This allows us to track down performance problems with this feature and
is generally a good idea.

This was possible earlier through debugfs, but only with special
debugging options set.  Also fix the boot message.

[akpm@linux-foundation.org: s/sched_numa_balancing/sysctl_numa_balancing/]
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0cd2c4d..947293e 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2668,7 +2668,7 @@
 
 	if (nr_node_ids > 1 && !numabalancing_override) {
 		printk(KERN_INFO "Enabling automatic NUMA balancing. "
-			"Configure with numa_balancing= or sysctl");
+			"Configure with numa_balancing= or the kernel.numa_balancing sysctl");
 		set_numabalancing_state(numabalancing_default);
 	}
 }