rsa/rsa_ossl.c: cache MONT_CTX for public modulus earlier.

Blinding is performed more efficiently and securely if MONT_CTX for public
modulus is available by the time blinding parameter are instantiated. So
make sure it's the case.
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7586)

(cherry picked from commit 2cc3f68cde77af23c61fbad65470602ee86f2575)

crypto/rsa/rsa_ossl.c

@@ -286,6 +286,11 @@ static int rsa_ossl_private_encrypt(int flen, const unsigned char *from,
         goto err;
     }
 
+    if (rsa->flags & RSA_FLAG_CACHE_PUBLIC)
+        if (!BN_MONT_CTX_set_locked(&rsa->_method_mod_n, rsa->lock,
+                                    rsa->n, ctx))
+            goto err;
+
     if (!(rsa->flags & RSA_FLAG_NO_BLINDING)) {
         blinding = rsa_get_blinding(rsa, &local_blinding, ctx);
         if (blinding == NULL) {
@@ -318,13 +323,6 @@ static int rsa_ossl_private_encrypt(int flen, const unsigned char *from,
         }
         BN_with_flags(d, rsa->d, BN_FLG_CONSTTIME);
 
-        if (rsa->flags & RSA_FLAG_CACHE_PUBLIC)
-            if (!BN_MONT_CTX_set_locked(&rsa->_method_mod_n, rsa->lock,
-                                        rsa->n, ctx)) {
-                BN_free(d);
-                goto err;
-            }
-
         if (!rsa->meth->bn_mod_exp(ret, f, d, rsa->n, ctx,
                                    rsa->_method_mod_n)) {
             BN_free(d);