LU-10468 tests: sync zfs dataset before reading blocks

LU-10468 tests: sync zfs dataset before reading blocks

Before reading blocks it should synchronize zfs dataset therefore
the block number will be accurate.

Test-Parameters: trivial envdefinitions=SLOW=yes,ENABLE_QUOTA=yes mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4 testlist=sanity-flr,sanity-flr,sanity-flr
Test-Parameters: trivial envdefinitions=SLOW=yes,ENABLE_QUOTA=yes mdtfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs mdscount=2 mdtcount=4 testlist=sanity-flr,sanity-flr,sanity-flr
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I663d689296e5847b460de9a491b551a56bfbc77d
Reviewed-on: https://review.whamcloud.com/30828
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>