compat_ioctl: move floppy handlers to block/compat_ioctl.c

The floppy ioctls are used by multiple drivers, so they should be
handled in a shared location. Also, add minor cleanups.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>