cleanup ubd gendisk registration

[Christoph Hellwig]

Hi all,

this series sits on top of Jens' for-5.14/block branch and tries to
convert ubd to the new gendisk and request_queue registration helpers.
As part of that I found that the ide emulation code currently registers
two gendisk for a request_queue which leads to a bunch of problems we've
avoided in other drivers (only the mmc subsystem has a similar issue).
Given that the legacy IDE driver isn't practically used any more and
modern userspace doesn't hard code specific block drivers, so I think
we can just drop it.  Let me know if this is ok.