btrfs: do not BUG_ON() on tree mod log failure at balance_level()

At balance_level(), instead of doing a BUG_ON() in case we fail to record
tree mod log operations, do a transaction abort and return the error to
the callers. There's really no need for the BUG_ON() as we can release
all resources in this context, and we have to abort because other future
tree searches that use the tree mod log (btrfs_search_old_slot()) may get
inconsistent results if other operations modify the tree after that
failure and before the tree mod log based search.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -1054,7 +1054,12 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 		}
 
 		ret = btrfs_tree_mod_log_insert_root(root->node, child, true);
-		BUG_ON(ret < 0);
+		if (ret < 0) {
+			btrfs_tree_unlock(child);
+			free_extent_buffer(child);
+			btrfs_abort_transaction(trans, ret);
+			goto enospc;
+		}
 		rcu_assign_pointer(root->node, child);
 
 		add_root_to_dirty_list(root);
@@ -1142,7 +1147,10 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 			btrfs_node_key(right, &right_key, 0);
 			ret = btrfs_tree_mod_log_insert_key(parent, pslot + 1,
 					BTRFS_MOD_LOG_KEY_REPLACE);
-			BUG_ON(ret < 0);
+			if (ret < 0) {
+				btrfs_abort_transaction(trans, ret);
+				goto enospc;
+			}
 			btrfs_set_node_key(parent, &right_key, pslot + 1);
 			btrfs_mark_buffer_dirty(parent);
 		}
@@ -1188,7 +1196,10 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 		btrfs_node_key(mid, &mid_key, 0);
 		ret = btrfs_tree_mod_log_insert_key(parent, pslot,
 						    BTRFS_MOD_LOG_KEY_REPLACE);
-		BUG_ON(ret < 0);
+		if (ret < 0) {
+			btrfs_abort_transaction(trans, ret);
+			goto enospc;
+		}
 		btrfs_set_node_key(parent, &mid_key, pslot);
 		btrfs_mark_buffer_dirty(parent);
 	}