LU-11721 lod: limit statfs ffree if less than OST ffree

LU-11721 lod: limit statfs ffree if less than OST ffree

If the OSTs report fewer total free objects than the MDTs, then
use the free files count reported by the OSTs, since it represents
the minimum number of files that can be created in the filesystem
(creating more may be possible, but this depends on other factors).
This has always been what ll_statfs_internal() reports, but the
statfs aggregation via the MDT missed this step in lod_statfs().

Fix a minor defect in sanity test_418() that would let it loop
forever until the test was killed due to timeout if the "df -i"
and "lfs df -i" output did not converge.

Fixes: b500d5193360 ("LU-10018 protocol: MDT as a statfs proxy")
Fixes: 263e80f4572b ("LU-11721 tests: wait for statfs to update ...")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Id8d7b7edfd854f1ec30bfbbb85f04b0c973ebbe5
Reviewed-on: https://review.whamcloud.com/34167
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Nikitas Angelinas <nangelinas@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>