LU-11721 tests: wait for statfs to update on DNE

LU-11721 tests: wait for statfs to update on DNE

Wait for the statfs to update properly when there are multiple
MDTs so that the test doesn't gratuitously fail.

Fixes: 757403191c3 ("LU-11721 utils: print used inodes ratio ...")
Test-Parameters: trivial testlist=sanity mdscount=2 mdtcount=4 ostcount=7
Test-Parameters: testlist=sanity fstype=zfs mdscount=2 mdtcount=4
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ia75f7bd4d3027c91f10ce990730b2bd7123ebbe5
Reviewed-on: https://review.whamcloud.com/34164
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>