crypto: api - Do not zap spawn->alg

Currently when a spawn is removed we will zap its alg field.
This is racy because the spawn could belong to an unregistered
instance which may dereference the spawn->alg field.

This patch fixes this by keeping spawn->alg constant and instead
adding a new spawn->dead field to indicate that a spawn is going
away.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/algapi.h

@@ -70,6 +70,7 @@ struct crypto_spawn {
 	struct crypto_instance *inst;
 	const struct crypto_type *frontend;
 	u32 mask;
+	bool dead;
 };
 
 struct crypto_queue {