LU-1282 lprocfs: fix multi-thread contention glitch

LU-1282 lprocfs: fix multi-thread contention glitch

* Revert clear stats patch committed on 8c831cb8, it is not multi-
  thread safe.
* Should protect the change of lprocfs_stats::ls_biggest_alloc_num
* Add a LPROCFS_STATS_FLAG_IRQ_SAFE flag, when a stat is non-percpu
  stats with this flag, lprocfs_stats_lock() should disable irq.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I42f29e97ff75bf3817249940c8bb491af123d1d9
Reviewed-on: http://review.whamcloud.com/3240
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>