btrfs-progs: tune convert-bgt: fix uninitialized value which leads to failed resume

[BUG]
There is a bug report that btrfstune failed to resume an interrupted
conversion.

I reproduced locally with an image which is half converted, in extent
tree there are 3 block groups:

	item 0 key (1048576 BLOCK_GROUP_ITEM 4194304) itemoff 3971 itemsize 24
	item 3 key (5242880 BLOCK_GROUP_ITEM 8388608) itemoff 3881 itemsize 24
	item 8 key (13631488 BLOCK_GROUP_ITEM 8388608) itemoff 3687 itemsize 24

And 4 block groups in the new block group tree:

	item 0 key (22020096 BLOCK_GROUP_ITEM 117440512) itemoff 3971 itemsize 24
	item 1 key (139460608 BLOCK_GROUP_ITEM 117440512) itemoff 3947 itemsize 24
	item 2 key (256901120 BLOCK_GROUP_ITEM 117440512) itemoff 3923 itemsize 24
	item 3 key (374341632 BLOCK_GROUP_ITEM 117440512) itemoff 3899 itemsize 24

Then resume the conversion will fail with the following error:

  $ btrfstune  --convert-to-block-group-tree /dev/test/scratch1
  ERROR: failed to find block group for bytenr 13631488
  ERROR: failed to convert the filesystem to block group tree feature
  ERROR: btrfstune failed
  extent buffer leak: start 23224320 len 4096

Meanwhile as the above dump tree shows, the block group item 13631488 is
indeed in the old extent tree.

[CAUSE]
During opening of such half-converted fs, btrfs will load the block
group items in two steps:

- read_old_block_groups_from_root()
  Which will load the block groups from the old tree (extent tree in the
  above case).

- read_block_groups_from_root()
  Which will load the block groups from the new tree (block group tree in
  the above case).

The problem is inside read_old_block_groups_from_root(), which uses an
on-stack @key to not only indicate where to start the search.

Unfortunately we didn't initialize that @key structure, resulting in
garbage initial value, which can be larger than all block group items
in the extent tree.

So these block group items are not loaded into our block group cache,
then leading to the conversion failure, as we rely on the block group
cache built at opening time.

[FIX]
Fix the uninitialized @key by initializing it to zero, as we expect to
search for the first block group item.

So that the block group items in the old tree can be properly loaded.

Reported-by: Tine Mezgec <tine.mezgec@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/a90b9418-48e8-47bf-8ec0-dd377a7c1f4e@gmail.com/
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

kernel-shared/extent-tree.c

@@ -2860,7 +2860,7 @@ static int read_old_block_groups_from_root(struct btrfs_fs_info *fs_info,
 					   struct btrfs_root *root)
 {
 	struct btrfs_path path = {0};
-	struct btrfs_key key;
+	struct btrfs_key key = { 0 };
 	struct cache_extent *ce;
 	/* The last block group bytenr in the old root. */
 	u64 last_bg_in_old_root;