LU-2877 tests: sanity test_34h needs to flush cache after write

We need to ensure that the cache is clean after dd in the test,
otherwise subsequent multiop might block trying to flush the
dirty pages for more than 2 seconds and trigger a false failure.

Change-Id: Ifb5a0aa0f9c627b353abe0d402c42a4e14d67609
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/5541
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>