ataflop: fix sleep_on races

sleep_on() is inherently racy, and has been deprecated for a long time.
This fixes two instances in the atari floppy driver:

* fdc_wait/fdc_busy becomes an open-coded mutex. We cannot use the
  regular mutex since it gets released in interrupt context. The
  open-coded version using wait_event() and cmpxchg() is equivalent
  to the existing code but does the checks atomically, and we can
  now safely check the condition with irqs enabled.

* format_wait becomes a completion, which is the natural structure
  here. The format ioctl waits for the background task to either
  complete or abort.

This does not attempt to fix the preexisting bug of calling schedule
with local interrupts disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Schmitz <schmitz@biophys.uni-duesseldorf.de>
Signed-off-by: Jens Axboe <axboe@fb.com>