LU-4858 ofd: statfs small fixes

ofd_osd is valid in its life time because ofd is released before
osd, so it should never be cleared.

revive o_statfs for ofd_obd_ops which was mistakenly removed in
commit I5622e14bccacb809bca1c10499c23bcaf72e2a68.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I3cb6d82b71b3018b4fc9e885eb6533b064179238
Reviewed-on: http://review.whamcloud.com/10082
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>