EX-3041 lipe: include reserved space into disk usage

EX-3041 lipe: include reserved space into disk usage

Typically ext4 fs reserves 5% disk space for root. It's
calculated as used space when df command shows the Use%.
So we calculate the usage in lpcc_purge in the same way
to be consistent with df.

Change-Id: I1cdee6ea66ad27bf7501cecb4a4a9495e09647a6
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/43376
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>