staging: lustre: 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.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>