nvmet: don't look at the request_queue in nvmet_bdev_set_limits

nvmet is a consumer of the block layer and should not directly look at
the request_queue.  Use the bdev_ helpers to retrieve the device limits
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>