LU-7418 tests: avoid background writeout in sanity 29

LU-7418 tests: avoid background writeout in sanity 29

Intermittent failures of sanity.sh test_29 are caused by background
writeout starting (or continuing) after the MDC DLM locks were
cancelled, requiring the layout lock to be re-fetched from the MDS.
Cancelling the DLM locks via "lru_size=clear" is not enough, since
this will not cancel any locks that are currently in use.

Instead, ensure that all the dirty pages from previous tests are
written out so that no new writes are started during the test.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ie45965a974e0cd170f969fa122e1762c2361c278
Reviewed-on: http://review.whamcloud.com/20929
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>