block/swim3: Locking fixes

The old PowerMac swim3 driver has some "interesting" locking issues,
using a private lock and failing to lock the queue before completing
requests, which triggered WARN_ONs among others.

This rips out the private lock, makes everything operate under the
block queue lock, and generally makes things simpler.

We used to also share a queue between the two possible instances which
was problematic since we might pick the wrong controller in some cases,
so make the queue and the current request per-instance and use
queuedata to point to our private data which is a lot cleaner.

We still share the queue lock but then, it's nearly impossible to actually
use 2 swim3's simultaneously: one would need to have a Wallstreet
PowerBook, the only machine afaik with two of these on the motherboard,
and populate both hotswap bays with a floppy drive (the machine ships
only with one), so nobody cares...

While at it, add a little fix to clear up stale interrupts when loading
the driver or plugging a floppy drive in a bay.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>