Btrfs: kill BUG_ON in run_delayed_tree_ref

Btrfs: kill BUG_ON in run_delayed_tree_ref

commit 02794222c4132ac003e7281fb71f4ec1645ffc87 upstream.

In a corrupted btrfs image, we can come across this BUG_ON and
get an unreponsive system, but if we return errors instead,
its caller can handle everything gracefully by aborting the current
transaction.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>