btrfs: use bdev_max_active_zones instead of open coding it

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220415045258.199825-9-hch@lst.de


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