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/akcipher.c b/crypto/akcipher.c
index eeed6c1..f866085 100644
--- a/crypto/akcipher.c
+++ b/crypto/akcipher.c
@@ -147,6 +147,8 @@ EXPORT_SYMBOL_GPL(crypto_unregister_akcipher);
 int akcipher_register_instance(struct crypto_template *tmpl,
 			       struct akcipher_instance *inst)
 {
+	if (WARN_ON(!inst->free))
+		return -EINVAL;
 	akcipher_prepare_alg(&inst->alg);
 	return crypto_register_instance(tmpl, akcipher_crypto_instance(inst));
 }