The driver allocates queues for all the units it potentially
supports. But if we fail to detect any drives, then we fail
loading the module without cleaning up those queues. This is
now evident with the switch to blk-mq, though the bug has
been there forever as far as I can tell.

Also fix cleanup through regular module exit.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>