Replace the use of one scan kthread per controller with one per driver.
Use a queue to hold a list of controllers that need to be rescanned with
routines to add and remove controllers from the queue.

Fix locking and completion handling to prevent a hang during rmmod.
Signed-off-by: NAndrew Patterson <andrew.patterson@hp.com>
Signed-off-by: NStephen M. Cameron <scameron@beardog.cce.hp.com>
Acked-by: NMike Miller <mike.miller@hp.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>