ubifs: Set double hash cookie also for RENAME_EXCHANGE

We developed RENAME_EXCHANGE and UBIFS_FLG_DOUBLE_HASH more or less in
parallel and this case was forgotten. :-(

Cc: stable@vger.kernel.org
Fixes: d63d61c1 ("ubifs: Implement UBIFS_FLG_DOUBLE_HASH")
Signed-off-by: Richard Weinberger <richard@nod.at>