Use the common helper function crypto_authenc_extractkeys() for key
parsing. Also ensure the auth key won't overflow the hash_ctx buffer.

Cc: Jamie Iles <jamie@jamieiles.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>