block: move the bounce flag into the features field

Move the bounce flag into the features field to reclaim a little bit of
space.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20240617060532.127975-27-hch@lst.de


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