LU-352 obdfilter: fix obdfilter-survey to work on verify mode.

With r/o cache landing lctl test_brw command needs to choose a type
of operation with backend device. osc needs to use obd_brw mode,
but obdfilter doesn't work in that mode. It needs to force to use a
obd_prep + obd_commit more.

Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-131, MRP-277
Change-Id: I562784b3e9fe537929e78e0197256949cb27a9e1
Reviewed-on: http://review.whamcloud.com/598
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>