ubifs: journal: Make sure to not dirty twice for auth nodes

When removing the last reference of an inode the size of an auth node
is already part of write_len. So we must not call ubifs_add_auth_dirt().
Call it only when needed.

Cc: <stable@vger.kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Kristof Havasi <havasiefr@gmail.com>
Fixes: 6a98bc46 ("ubifs: Add authentication nodes to journal")
Reported-and-tested-by: Kristof Havasi <havasiefr@gmail.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>