btrfs-progs: mkfs: fix a crash when enabling extent-tree-v2

[BUG]
When enabling extent-tree-v2 feature at mkfs time (need to enable
experimental features), mkfs.btrfs will crash:

  # ./mkfs.btrfs  -f -O extent-tree-v2 ~/test.img
  btrfs-progs v5.19.1
  See http://btrfs.wiki.kernel.org for more information.

  ERROR: superblock magic doesn't match
  NOTE: several default settings have changed in version 5.15, please make sure
        this does not affect your deployments:
        - DUP for metadata (-m dup)
        - enabled no-holes (-O no-holes)
        - enabled free-space-tree (-R free-space-tree)

  Segmentation fault (core dumped)

[CAUSE]
The block group tree looks like this after make_btrfs() call:

  (gdb) call btrfs_print_tree(root->fs_info->block_group_root->node, 0)
  leaf 1163264 items 1 free space 16234 generation 1 owner BLOCK_GROUP_TREE
  leaf 1163264 flags 0x0() backref revision 1
  checksum stored f137c1ac
  checksum calced f137c1ac
  fs uuid 450d4b15-4954-4574-9801-8c6d248aaec6
  chunk uuid 4c4cc54d-f240-4aa4-b88b-bd487db43444
	item 0 key (1048576 BLOCK_GROUP_ITEM 4194304) itemoff 16259 itemsize 24
		block group used 131072 chunk_objectid 256 flags SYSTEM|single
						       ^^^

This looks completely sane, but notice that chunk_objectid 256.
That 256 value is the expected one for regular non-extent-tree-v2 btrfs,
but for extent-tree-v2, chunk_objectid is reused as the global id of
extent tree where the block group belongs to.

With the old 256 value as chunk_objectid, btrfs will not find an extent
tree root for the block group, and return NULL for btrfs_extent_root()
call, and trigger segfault.

This is a regression caused by commit 1430b41 ("btrfs-progs:
separate block group tree from extent tree v2"), which doesn't take
extent-tree-v2 on-disk format into consideration.

[FIX]
For the initial btrfs created by make_btrfs(), all block group items
will be in extent-tree global id 0, thus we can reset chunk_objectid to
0, if and only if extent-tree-v2 is enabled.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Tested-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

mkfs/common.c

@@ -227,12 +227,22 @@ static int create_block_group_tree(int fd, struct btrfs_mkfs_config *cfg,
 				   u64 bg_offset, u64 bg_size, u64 bg_used)
 {
 	int ret;
+	u64 chunk_objectid = BTRFS_FIRST_CHUNK_TREE_OBJECTID;
+
+	/*
+	 * For extent-tree-v2, chunk_objectid of block group item is reused
+	 * to indicate which extent-tree the block group is in.
+	 *
+	 * Thus for the initial image, we should set the chunk_objectid to 0,
+	 * as all initial bgs are in the extent tree with global id 0.
+	 */
+	if (cfg->features.incompat_flags & BTRFS_FEATURE_INCOMPAT_EXTENT_TREE_V2)
+		chunk_objectid = 0;
 
 	memset(buf->data + sizeof(struct btrfs_header), 0,
 		cfg->nodesize - sizeof(struct btrfs_header));
 	write_block_group_item(buf, 0, bg_offset, bg_size, bg_used,
-			       BTRFS_FIRST_CHUNK_TREE_OBJECTID,
-			       cfg->leaf_data_size -
+			       chunk_objectid, cfg->leaf_data_size -
 			       sizeof(struct btrfs_block_group_item));
 	btrfs_set_header_bytenr(buf, cfg->blocks[MKFS_BLOCK_GROUP_TREE]);
 	btrfs_set_header_owner(buf, BTRFS_BLOCK_GROUP_TREE_OBJECTID);