LU-4858 ofd: ofd_osd is always valid

LU-4858 ofd: ofd_osd is always valid

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

Lustre-commit: I3cb6d82b71b3018b4fc9e885eb6533b064179238
Lustre-change: http://review.whamcloud.com/10082

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I2ef4ba4a8faeaf8b718c9888332fa180c144d68b
Reviewed-on: http://review.whamcloud.com/10084
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>