btrfs: use chunk_root in find_free_extent_update_loop

We're only using this to start the transaction with to possibly allocate
a chunk.  It doesn't really matter which root to use, but with extent
tree v2 we'll need a bytenr to look up a extent root which makes the
usage of the extent_root awkward here.  Simply change it to the
chunk_root.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -3977,7 +3977,7 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
 					struct find_free_extent_ctl *ffe_ctl,
 					bool full_search)
 {
-	struct btrfs_root *root = fs_info->extent_root;
+	struct btrfs_root *root = fs_info->chunk_root;
 	int ret;
 
 	if ((ffe_ctl->loop == LOOP_CACHING_NOWAIT) &&