DH: have DH_set_length() increment the dirty count.

The recommended private key length is a key parameter among other key
parameters, and is included in the key data transferred in an import
or export between legacy implementations and provider implementations.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13166)

crypto/dh/dh_lib.c

@@ -243,6 +243,7 @@ long DH_get_length(const DH *dh)
 int DH_set_length(DH *dh, long length)
 {
     dh->length = length;
+    dh->dirty_cnt++;
     return 1;
 }