LU-12248 lov: fix ost objects calculation in lod_statfs

LU-12248 lov: fix ost objects calculation in lod_statfs

Wen OSTs report fewer free objects than MDTs, the statfs
objects results are presented with the numbers reported
by OSTs. Fix the calculation of OST objetcs to make it
work with statfs aggregation via the MDT.

Make the lfs code consistent with ll_statfs_internal()
and lod_statfs().

Fixes: a829595add ("LU-11721 lod: limit statfs ffree if less ...")

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I838a1527ed6411a412b63e2855ca7247755a3bcf
Reviewed-on: https://review.whamcloud.com/34777
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>