LU-7297 osd-zfs: initialize oh_lock

LU-7297 osd-zfs: initialize oh_lock

The ZFS osd was not initializing od_brw_stats.hist[].oh_lock.
This rectifies that.

Change-Id: I3f637b73c77908c2297bfab97e33eca63b0d5986
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/16919
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>