esp, ah: consolidate the crypto algorithm selections

Instead of duplicating the algorithm selections between INET_AH and
INET6_AH and between INET_ESP and INET6_ESP, create new tristates
XFRM_AH and XFRM_ESP that do the algorithm selections, and make these be
selected by the corresponding INET* options.

Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Corentin Labbe <clabbe@baylibre.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>