s390/crypto: Fix kernel crash on aes_s390 module remove.

A kernel crash occurs when the aes_s390 kernel module is
removed on machines < z14. This only happens on kernel
version 4.15 and higher on machines not supporting MSA 8.

The reason for the crash is a unconditional
crypto_unregister_aead() invocation where no previous
crypto_register_aead() had been called. The fix now
remembers if there has been a successful registration and
only then calls the unregister function upon kernel module
remove.

The code now crashing has been introduced with
"bf7fa038 s390/crypto: add s390 platform specific aes gcm support."
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/crypto/aes_s390.c

@@ -1047,6 +1047,7 @@ static struct aead_alg gcm_aes_aead = {
 
 static struct crypto_alg *aes_s390_algs_ptr[5];
 static int aes_s390_algs_num;
+static struct aead_alg *aes_s390_aead_alg;
 
 static int aes_s390_register_alg(struct crypto_alg *alg)
 {
@@ -1065,7 +1066,8 @@ static void aes_s390_fini(void)
 	if (ctrblk)
 		free_page((unsigned long) ctrblk);
 
-	crypto_unregister_aead(&gcm_aes_aead);
+	if (aes_s390_aead_alg)
+		crypto_unregister_aead(aes_s390_aead_alg);
 }
 
 static int __init aes_s390_init(void)
@@ -1123,6 +1125,7 @@ static int __init aes_s390_init(void)
 		ret = crypto_register_aead(&gcm_aes_aead);
 		if (ret)
 			goto out_err;
+		aes_s390_aead_alg = &gcm_aes_aead;
 	}
 
 	return 0;