xfrm: Restrict extended sequence numbers to esp

The IPsec extended sequence numbers are fully implemented just for
esp. So restrict the usage to esp until other protocols have
support too.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>