ublk driver takes the following ioctl numbers:

	'u'   00-2F  linux/ublk_cmd.h

So document it in ioctl-number.rst
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240823134339.1496916-1-ming.lei@redhat.com