crypto: move gf128mul library into lib/crypto

The gf128mul library does not depend on the crypto API at all, so it can
be moved into lib/crypto. This will allow us to use it in other library
code in a subsequent patch without having to depend on CONFIG_CRYPTO.

While at it, change the Kconfig symbol name to align with other crypto
library implementations. However, the source file name is retained, as
it is reflected in the module .ko filename, and changing this might
break things for users.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>