blkdev: make struct block_device_operations.devnode() take a const *

The devnode() callback in struct block_device_operations should not be
modifying the device that is passed into it, so mark it as a const * and
propagate the function signature changes out into the one subsystem that
actually uses this callback.

Acked-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20221109144843.679668-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>