block: don't return 1 for the fallback case in blkdev_get_zone_info

block: don't return 1 for the fallback case in blkdev_get_zone_info

blkdev_do_report_zones returns the number of reported zones, but
blkdev_get_zone_info returns 0 or an errno.  Translate to the expected
return value in blkdev_report_zone_fallback.

Fixes: b037d41762fd ("block: introduce blkdev_get_zone_info()")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>