btrfs: relocation: Only remove reloc rb_trees if reloc control has been initialized

Invalid reloc tree can cause kernel NULL pointer dereference when btrfs
does some cleanup of the reloc roots.

It turns out that fs_info::reloc_ctl can be NULL in
btrfs_recover_relocation() as we allocate relocation control after all
reloc roots have been verified.
So when we hit: note, we haven't called set_reloc_control() thus
fs_info::reloc_ctl is still NULL.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=199833Reported-by: Xu Wen <wen.xu@gatech.edu>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Tested-by: Gu Jinxiang <gujx@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -1281,6 +1281,7 @@ static void __del_reloc_root(struct btrfs_root *root)
 	struct mapping_node *node = NULL;
 	struct reloc_control *rc = fs_info->reloc_ctl;
 
+	if (rc) {
 		spin_lock(&rc->reloc_root_tree.lock);
 		rb_node = tree_search(&rc->reloc_root_tree.rb_root,
 				      root->node->start);
@@ -1289,10 +1290,10 @@ static void __del_reloc_root(struct btrfs_root *root)
 			rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
 		}
 		spin_unlock(&rc->reloc_root_tree.lock);
-
 		if (!node)
 			return;
 		BUG_ON((struct btrfs_root *)node->data != root);
+	}
 
 	spin_lock(&fs_info->trans_lock);
 	list_del_init(&root->root_list);