LU-3700 tests: zfs: work around blocks check for sanity-hsm/21

The number of blocks for ZFS reflects the number of blocks
written to disk.  To get an accurate number, the file on the OST needs
to be fully flushed.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I99ffac888a80b046b8c446021124e92f0f313f27
Reviewed-on: http://review.whamcloud.com/8575
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>