LU-11771 osd: avoid use of HZ in brw_stats

LU-11771 osd: avoid use of HZ in brw_stats

Don't use HZ when computing the brw_stats IO time.  This brw_stats
field was formerly in jiffies, but since the conversion to ktime_t
(now always in msec) the output is incorrect when HZ != 1000.

Fixes: 4c2f028a95063aa30006a071a039e8a3eefe52f2

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ia01705b94ddedec2e375621c6e01a957f53ebbe5
Reviewed-on: https://review.whamcloud.com/33857
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>