block: make blk_set_default_limits() private

There are no external users of this function.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221110184501.2451620-4-kbusch@meta.com


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