crypto: rsa-pkcs1pad - only allow with rsa

The pkcs1pad template can be instantiated with an arbitrary akcipher
algorithm, which doesn't make sense; it is specifically an RSA padding
scheme.  Make it check that the underlying algorithm really is RSA.

Fixes: 3d5b1ecd

 ("crypto: rsa - RSA padding algorithm")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>