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

At insert_new_root(), instead of doing a BUG_ON() in case we fail to
record the tree mod log operation, just return the error to the callers
after releasing the allocated tree block. At this point we haven't made
any changes to the b+tree, so we haven't left it in an inconsistent state
and therefore have no need to abort the transaction. All we need to do is
to unlock and free the extent buffer we just allocated with the purpose
of making it the new root.
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

@@ -2964,7 +2964,12 @@ static noinline int insert_new_root(struct btrfs_trans_handle *trans,
 
 	old = root->node;
 	ret = btrfs_tree_mod_log_insert_root(root->node, c, false);
-	BUG_ON(ret < 0);
+	if (ret < 0) {
+		btrfs_free_tree_block(trans, btrfs_root_id(root), c, 0, 1);
+		btrfs_tree_unlock(c);
+		free_extent_buffer(c);
+		return ret;
+	}
 	rcu_assign_pointer(root->node, c);
 
 	/* the super has an extra ref to root->node */