LU-4423 obdclass: use workqueue for zombie management

obdclass currently maintains two lists of data structures (imports and
exports), and a kthread which will free anything on either list.  The
thread is woken whenever anything is added to either list.

This is exactly the sort of thing that workqueues exist for.

So discard the zombie kthread and the lists and locks, and create a
single workqueue.  Each obd_import and obd_export gets a work_struct to
attach to this workqueue.

This requires a small change to import_sec_validate_get() which was
testing if an obd_import was on the zombie list.  This cannot have every
safely found it to be on the list (as it could be freed asynchronously)
so it must be dead code.

We could use system_wq instead of creating a dedicated zombie_wq, but as
we occasionally want to flush all pending work, it is a little nicer to
only have to wait for our own work items.

Change-Id: I3d0fafcc4d3896ff8760d5d36d8cfa187f86bc7d
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: https://review.whamcloud.com/31707
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>