nbd: add support for rotational devices

The NBD protocol defines the flag NBD_FLAG_ROTATIONAL to flag that the
export in use should be treated as a rotational device.

Add support for that flag to the kernel driver.

Signed-off-by: Wouter Verhelst <w@uter.be>
Reviewed-by: Eric Blake <eblake@redhat.com>
Link: https://lore.kernel.org/r/20240725164536.1275851-1-w@uter.be


Signed-off-by: Jens Axboe <axboe@kernel.dk>