[IPSEC] ah: Remove keys from ah_data structure

The keys are only used during initialisation so we don't need to carry them
in esp_data.  Since we don't have to allocate them again, there is no need
to place a limit on the authentication key length anymore.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>