crypto: algapi - reject NULL crypto_spawn::inst

It took me a while to notice the bug where the adiantum template left
crypto_spawn::inst == NULL, because this only caused problems in certain
cases where algorithms are dynamically loaded/unloaded.

More improvements are needed, but for now make crypto_init_spawn()
reject this case and WARN(), so this type of bug will be noticed
immediately in the future.

Note: I checked all callers and the adiantum template was the only place
that had this wrong.  So this WARN shouldn't trigger anymore.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/algapi.c

@@ -608,6 +608,9 @@ int crypto_init_spawn(struct crypto_spawn *spawn, struct crypto_alg *alg,
 {
 	int err = -EAGAIN;
 
+	if (WARN_ON_ONCE(inst == NULL))
+		return -EINVAL;
+
 	spawn->inst = inst;
 	spawn->mask = mask;