Linux 6.11: get backing_dev_info through queue gendisk

It's no longer available directly on the request queue, but its easy to
get from the attached disk.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16400