crypto: shash - eliminate indirect call for default import and export

Most shash algorithms don't have custom ->import and ->export functions,
resulting in the memcpy() based default being used.  Yet,
crypto_shash_import() and crypto_shash_export() still make an indirect
call, which is expensive.  Therefore, change how the default import and
export are called to make it so that crypto_shash_import() and
crypto_shash_export() don't do an indirect call in this case.

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