crypto: algapi - enforce that all instances have a ->free() method

All instances need to have a ->free() method, but people could forget to
set it and then not notice if the instance is never unregistered.  To
help detect this bug earlier, don't allow an instance without a ->free()
method to be registered, and complain loudly if someone tries to do it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/ahash.c b/crypto/ahash.c
index cd5d984..68a0f0c 100644
--- a/crypto/ahash.c
+++ b/crypto/ahash.c
@@ -656,6 +656,9 @@ int ahash_register_instance(struct crypto_template *tmpl,
 {
 	int err;
 
+	if (WARN_ON(!inst->free))
+		return -EINVAL;
+
 	err = ahash_prepare_alg(&inst->alg);
 	if (err)
 		return err;