btrfs: add a leak check for roots

Now that we're going to start relying on getting ref counting right for
roots, add a list to track allocated roots and print out any roots that
aren't freed up at free_fs_info time.

Hide this behind CONFIG_BTRFS_DEBUG because this will just be used for
developers to verify they aren't breaking things.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
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/ctree.h

@@ -947,6 +947,7 @@ struct btrfs_fs_info {
 #ifdef CONFIG_BTRFS_DEBUG
 	struct kobject *debug_kobj;
 	struct kobject *discard_debug_kobj;
+	struct list_head allocated_roots;
 #endif
 };
 
@@ -1149,6 +1150,10 @@ struct btrfs_root {
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 	u64 alloc_bytenr;
 #endif
+
+#ifdef CONFIG_BTRFS_DEBUG
+	struct list_head leak_list;
+#endif
 };
 
 struct btrfs_clone_extent_info {