LU-4482 grant: don't use cache data in osd_statfs()

LU-4482 grant: don't use cache data in osd_statfs()

osd_statfs() shouldn't cache statfs data anymore: the statfs data
is already cached in ofd layer, put another cache in osd layer
looks redundant, and what more important is: grant mechanism relies
on dt_statfs() returning fresh statfs data, caching statfs data in
osd layer would just break grant.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I89b6384cc59d77b1edb0412f24b5c8e823532170
Reviewed-on: http://review.whamcloud.com/8911
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>