btrfs: zoned: fix uninitialized variable warning in btrfs_get_dev_zones

Fix an uninitialized warning we get with -Wmaybe-uninitialized where it
thought zno may have been uninitialized, in both cases it depends on
zinfo->zone_cache but we know the value won't change between checks.

Reported-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/linux-btrfs/af6c527cbd8bdc782e50bd33996ee83acc3a16fb.1671221596.git.josef@toxicpanda.com/


Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>