LU-9859 libcfs: remove wi_data from cfs_workitem

In every case, the value passed via wi_data can be determined
from the cfs_workitem pointer using container_of().

So use container_of(), and discard wi_data.

Linux-commit: 19ae89d32503493315dec77919815d3add851389

Change-Id: Iefc4b6ebf40b48bd60a5820de05eb44746a041c0
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/34466
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>