hpre: optimize key process before free

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

In this patch, we try to optimize key process before free of hpre
module.
1、for DH algorithm, we use para g to compute public key, para g is
public, so we need not clear it before free.
2、for RSA algorithm, we should use memzero_explicit to clear prikey.
It will be optimized, when memset follows free.
3、For standard RSA algorithm, we use one buf to store public and
prikey, to consider performance, we just clear prikey buf.

Signed-off-by: Yu'an Wang <wangyuan46@huawei.com>
Reviewed-by: Cheng Hu <hucheng.hu@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>