LU-1415 tests: Cope with ZFS's delayed space updates

LU-1415 tests: Cope with ZFS's delayed space updates

Space-related updates, such as changes of dn_used and of
ds_used_bytes, are invisible to opertions like dt_attr_get() and
dt_statfs() until after corresponding transaction groups start to be
synchronized.  This patch makes sure the tests do a dt_sync() for each
OST before checking space usage.

Change-Id: Icff5106c0c4c493ef981bc14a0aabac8da16764d
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/2909
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>