swim: convert to blk-mq

swim: convert to blk-mq

The only interesting thing here is that there may be two floppies (i.e.,
request queues) sharing the same controller, so we use the global struct
swim_priv->lock to check whether the controller is busy. Compile-tested
only.

Tested-by: Finn Thain <[email protected]>
Acked-by: Laurent Vivier <[email protected]>
Signed-off-by: Omar Sandoval <[email protected]>
Converted to blk_mq_init_sq_queue()

Signed-off-by: Jens Axboe <[email protected]>