fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

[Mike Galbraith]

Hi Christoph,

While you're fixing that commit up, can you perhaps find a better home
for quiet_vmstat()?  It not only munches cycles when switching cross
-core mightily, for -rt it injects a sleeping lock into the idle task.

    12.89%  [kernel]       [k] refresh_cpu_vm_stats.isra.12
     4.75%  [kernel]       [k] __schedule                  
     4.70%  [kernel]       [k] mutex_unlock                
     3.14%  [kernel]       [k] __switch_to                 

        -Mike