stop using ioctl_by_bdev in the s390 DASD driver

[Christoph Hellwig]

Hi Jens and DASD maintainers,

can you take a look at this series, which stops the DASD driver from
issuing ioctls from kernel space, in preparation of removing
ioctl_by_bdev.  I don't really like the new s390-only method, but short
of forcing the dasd driver to be built into the kernel I can't think of
anything better.  But maybe the s390 maintainers are fine with forcing
the DASD driver to be built in, in which case we could go down that
route?