There is only one implementation of this ioctl, so move the handling out
of the common block layer code into the place where it's actually needed.

It also gets called indirectly through pktcdvd, which needs to be aware
of this change.

As I noticed, the old implementation of the compat handler failed to
convert the structure on the way out, so the updated fields never got
written back to user space. This is either not important, or it has
never worked and should be fixed now.
Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>