LU-7883 llog: Flush out journal callback data

LU-7883 llog: Flush out journal callback data

As LU-7329, llog_test_10 trigger OOM again. So add
dt_sync on subtest beginning to flush out journal
callback data accumulated by previous subtest.

Test-Parameters: envdefinitions=ONLY=60a testlist=sanity,sanity,sanity,sanity,sanity,sanity

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I25c3a5d5a6184b4f51f29948af9c3c0dff1abfaa
Reviewed-on: http://review.whamcloud.com/19005
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>