From: Geoff Gustafson <geoff@linux.jf.intel.com>,
      "Chen, Kenneth W" <kenneth.w.chen@intel.com>,
      Ingo Molnar <mingo@elte.hu>,
      me.

The big-SMP guys are seeing high CPU load due to del_timer_sync()'s
inefficiencies.  The callers are fs/aio.c and schedule_timeout().

We note that neither of these callers' timer handlers actually re-add the
timer - they are single-shot.

So we don't need all that complexity in del_timer_sync() - we can just run
del_timer() and if that worked we know the timer is dead.

Add del_single_shot_timer(), export it to modules and use it in AIO and
schedule_timeout().


(these numbers are for an earlier patch, but they'll be close)

Before:             32p     4p
     Warm cache   29,000    505
     Cold cache   37,800   1220

After:              32p     4p
     Warm cache       95     88
     Cold cache    1,800    140

[Measurements are CPU cycles spent in a call to del_timer_sync, the average
of 1000 calls. 32p is 16-node NUMA, 4p is SMP.]

(I cleaned up a few things and added some commentary)