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 <fthain@telegraphics.com.au>
Acked-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>

Converted to blk_mq_init_sq_queue()

Signed-off-by: Jens Axboe <axboe@kernel.dk>