compat_ioctl: add compat_blkdev_driver_ioctl()

Handle those blockdev ioctl calls that are compatible
directly from the compat_blkdev_ioctl() function, instead
of having to go through the compat_ioctl hash lookup.

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