crypto: rsassa-pkcs1 - Avoid copying hash prefix

When constructing the EMSA-PKCS1-v1_5 padding for the sign operation,
a buffer for the padding is allocated and the Full Hash Prefix is copied
into it.  The padding is then passed to the RSA decrypt operation as an
sglist entry which is succeeded by a second sglist entry for the hash.

Actually copying the hash prefix around is completely unnecessary.
It can simply be referenced from a third sglist entry which sits
in-between the padding and the digest.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>