HPRE can only deal with src_data smaller than 'p' in 'CURVE25519'
algorithm, but 'rfc7748' says:
'Implementations MUST accept non-canonical values and process them as
if they had been reduced modulo the field prime'
So we get its modulus to p, and then deal it with HPRE.
Signed-off-by: NMeng Yu <yumeng18@huawei.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>