LU-9859 libcfs: use a workqueue for rehash work.

LU-9859 libcfs: use a workqueue for rehash work.

lustre has a work-item queuing scheme that provides the
same functionality as linux work_queues.
To make the code easier for linux devs to follow, change
to use work_queues.

Linux-commit: 0aa211e39857f17e24126c47f6e3fe3b971344b3

Change-Id: I1600ea1ef8769f1f6489b81fd578685ea58f9cb6
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/34169
Tested-by: Jenkins
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>