compat_ioctl: ubd, aoe: use blkdev_compat_ptr_ioctl

These drivers implement the HDIO_GET_IDENTITY and CDROMVOLREAD ioctl
commands, which are compatible between 32-bit and 64-bit user space and
traditionally handled by compat_blkdev_driver_ioctl().

As a prerequisite to removing that function, make both drivers use
blkdev_compat_ptr_ioctl() as their .compat_ioctl callback.

Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>