ubifs: ensure zero err is returned on successful return

err is no longer being set on a successful return path, causing
a garbage value being returned. Fix this by setting err to zero
for the successful return path.

Found with static analysis by CoverityScan, CID 1389473

Fixes: 7799953b ("ubifs: Implement encrypt/decrypt for all IO")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Richard Weinberger <richard@nod.at>