MRP-1347 ost: deadlock with system flush

Writeback truncated page unter transaction lock.
It prevets flush from taking truncated page

Change-Id: I9f1f11cbafd2bebfdb661f1cac172388f1f60f76
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/670
Tested-by: Jenkins
Reviewed-by: Cheng Shao <cheng_shao@xyratex.com>
Reviewed-by: Alexey Lyashkov <Alexey_Lyashkov@xyratex.com>
Tested-by: Alexander Lezhoev <Alexander_Lezhoev@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>