crypto: gf128mul - Zero memory when freeing multiplication table

GF(2^128) multiplication tables are typically used for secret
information, so it's a good idea to zero them on free.

Signed-off-by: Alex Cope <alexcope@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>