btrfs: zoned: fix initial free space detection

When creating a new block group, it calls btrfs_add_new_free_space() to add
the entire block group range into the free space accounting.
__btrfs_add_free_space_zoned() checks if size == block_group->length to
detect the initial free space adding, and proceed that case properly.

However, if the zone_capacity == zone_size and the over-write speed is fast
enough, the entire zone can be over-written within one transaction. That
confuses __btrfs_add_free_space_zoned() to handle it as an initial free
space accounting. As a result, that block group becomes a strange state: 0
used bytes, 0 zone_unusable bytes, but alloc_offset == zone_capacity (no
allocation anymore).

The initial free space accounting can properly be checked by checking
alloc_offset too.

Fixes: 98173255 ("btrfs: zoned: calculate free space from 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>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -2697,7 +2697,7 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 	u64 offset = bytenr - block_group->start;
 	u64 to_free, to_unusable;
 	int bg_reclaim_threshold = 0;
-	bool initial = (size == block_group->length);
+	bool initial = ((size == block_group->length) && (block_group->alloc_offset == 0));
 	u64 reclaimable_unusable;
 
 	WARN_ON(!initial && offset + size > block_group->zone_capacity);