[SCSI] fcoe: use dedicated workqueue instead of system_wq

fcoe uses the system_wq to destroy ports and the work items need to be
flushed before the driver is unloaded.  As the work items free the
containing data structure, they can't be flushed directly.  The
workqueue should be flushed instead.

Also, the destruction works can be chained - ie. destruction of a port
may lead to destruction of another port where the work item for the
former queues the work for the latter.  Currently, the depth of chain
can be at most two and fcoe_exit() makes sure everything is complete
by calling flush_scheduled_work() twice.

With commit c8efcc25 (workqueue: allow chained queueing during
destruction), destroy_workqueue() can take care of chained works on
workqueue destruction.  Add and use fcoe_wq instead.  Simply
destroying fcoe_wq on driver unload takes care of flushing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>