btrfs: zoned: fix full zone super block reading on ZNS

When both of the superblock zones are full, we need to check which
superblock is newer. The calculation of last superblock position is wrong
as it does not consider zone_capacity and uses the length.

Fixes: 9658b72e

 ("btrfs: zoned: locate superblock position using zone capacity")
CC: stable@vger.kernel.org # 6.1+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>