crypto: virtio - Simplify RSA key size caching

crypto: virtio - Simplify RSA key size caching

When setting a public or private RSA key, the integer n is cached in the
transform context virtio_crypto_akcipher_ctx -- with the sole purpose of
calculating the key size from it in virtio_crypto_rsa_max_size().
It looks like this was copy-pasted from crypto/rsa.c.

Cache the key size directly instead of the integer n, thus simplifying
the code and reducing the memory footprint.

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