rbd: enable multiple blk-mq queues

Allocate one queue per CPU and get a performance boost from
higher parallelism.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>