Fix the default digest algorthm of SM2

Currently SM2 shares the ameth with EC, so the current default digest
algorithm returned is SHA256. This fixes the default digest algorithm of
SM2 to SM3, which is the only valid digest algorithm for SM2 signature.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8186)

(cherry picked from commit e766f4a0531bffdab8ad2038279b755928d7a40a)

crypto/ec/ec_ameth.c

@@ -504,7 +504,12 @@ static int ec_pkey_ctrl(EVP_PKEY *pkey, int op, long arg1, void *arg2)
 #endif
 
     case ASN1_PKEY_CTRL_DEFAULT_MD_NID:
-        *(int *)arg2 = NID_sha256;
+        if (EVP_PKEY_id(pkey) == EVP_PKEY_SM2) {
+            /* For SM2, the only valid digest-alg is SM3 */
+            *(int *)arg2 = NID_sm3;
+        } else {
+            *(int *)arg2 = NID_sha256;
+        }
         return 1;
 
     case ASN1_PKEY_CTRL_SET1_TLS_ENCPT: