crypto: shash - require neither or both ->export() and ->import()

Prevent registering shash algorithms that implement ->export() but not
->import(), or ->import() but not ->export().  Such cases don't make
sense and could confuse the check that shash_prepare_alg() does for just
->export().

I don't believe this affects any existing algorithms; this is just
preventing future mistakes.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/shash.c

@@ -472,6 +472,9 @@ static int shash_prepare_alg(struct shash_alg *alg)
 	    alg->statesize > HASH_MAX_STATESIZE)
 		return -EINVAL;
 
+	if ((alg->export && !alg->import) || (alg->import && !alg->export))
+		return -EINVAL;
+
 	base->cra_type = &crypto_shash_type;
 	base->cra_flags &= ~CRYPTO_ALG_TYPE_MASK;
 	base->cra_flags |= CRYPTO_ALG_TYPE_SHASH;