btrfs: annotate unlikely branches after V0 extent type removal

The v0 extent type checks are the right case for the unlikely
annotations as we don't expect to ever see them, so let's give the
compiler some hint.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/print-tree.c b/fs/btrfs/print-tree.c
index cc94537..df49931 100644
--- a/fs/btrfs/print-tree.c
+++ b/fs/btrfs/print-tree.c
@@ -52,7 +52,7 @@ static void print_extent_item(struct extent_buffer *eb, int slot, int type)
 	u64 offset;
 	int ref_index = 0;
 
-	if (item_size < sizeof(*ei)) {
+	if (unlikely(item_size < sizeof(*ei))) {
 		btrfs_print_v0_err(eb->fs_info);
 		btrfs_handle_fs_error(eb->fs_info, -EINVAL, NULL);
 	}