LU-1812 osd-ldiskfs: handle 4-arg fsync properly

LU-1812 osd-ldiskfs: handle 4-arg fsync properly

Fix locking around fsync in osd_object_sync.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Change-Id: Idc35c1649af7ef7d65fba316ffbcd86574fa847a
Reviewed-on: http://review.whamcloud.com/5115
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: chas williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>