btrfs: change BUG_ON to assertion in tree_move_down()

There's only one caller of tree_move_down() that does not pass level 0
so the assertion is better suited here.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -7447,8 +7447,8 @@ static int tree_move_down(struct btrfs_path *path, int *level, u64 reada_min_gen
 	u64 reada_done = 0;
 
 	lockdep_assert_held_read(&parent->fs_info->commit_root_sem);
+	ASSERT(*level != 0);
 
-	BUG_ON(*level == 0);
 	eb = btrfs_read_node_slot(parent, slot);
 	if (IS_ERR(eb))
 		return PTR_ERR(eb);