Documentation: add ublk driver ioctl numbers

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

Documentation/userspace-api/ioctl/ioctl-number.rst

@@ -293,6 +293,7 @@ Code  Seq#    Include File                                           Comments
 't'   80-8F  linux/isdn_ppp.h
 't'   90-91  linux/toshiba.h                                         toshiba and toshiba_acpi SMM
 'u'   00-1F  linux/smb_fs.h                                          gone
+'u'   00-2F  linux/ublk_cmd.h                                        conflict!
 'u'   20-3F  linux/uvcvideo.h                                        USB video class host driver
 'u'   40-4f  linux/udmabuf.h                                         userspace dma-buf misc device
 'v'   00-1F  linux/ext2_fs.h                                         conflict!