LU-6142 libcfs: remove last use of container_of0

LU-6142 libcfs: remove last use of container_of0

Remove the last use of container_of0 in osd_ldiskfs. Looking at
the code using container_of is safe to use since the handle
shouldn't be NULL or PTR_ERR.

Change-Id: Id018543997884825d1c3cee76de3a520fc3b7c18
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/39321
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>