Btrfs: always clear a block group node when removing it from the tree

Always clear a block group's rbnode after removing it from the rbtree to
ensure that any tasks that might be holding a reference on the block group
don't end up accessing stale rbnode left and right child pointers through
next_block_group().

This is a leftover from the change titled:
"Btrfs: fix invalid block group rbtree access after bg is removed"
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -8872,6 +8872,7 @@ int btrfs_free_block_groups(struct btrfs_fs_info *info)
 				       cache_node);
 		rb_erase(&block_group->cache_node,
 			 &info->block_group_cache_tree);
+		RB_CLEAR_NODE(&block_group->cache_node);
 		spin_unlock(&info->block_group_cache_lock);
 
 		down_write(&block_group->space_info->groups_sem);
@@ -9130,6 +9131,7 @@ int btrfs_read_block_groups(struct btrfs_root *root)
 			spin_lock(&info->block_group_cache_lock);
 			rb_erase(&cache->cache_node,
 				 &info->block_group_cache_tree);
+			RB_CLEAR_NODE(&cache->cache_node);
 			spin_unlock(&info->block_group_cache_lock);
 			btrfs_put_block_group(cache);
 			goto error;
@@ -9271,6 +9273,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans,
 		spin_lock(&root->fs_info->block_group_cache_lock);
 		rb_erase(&cache->cache_node,
 			 &root->fs_info->block_group_cache_tree);
+		RB_CLEAR_NODE(&cache->cache_node);
 		spin_unlock(&root->fs_info->block_group_cache_lock);
 		btrfs_put_block_group(cache);
 		return ret;