crypto: authencesn - Fix breakage with new ESP code

The ESP code has been updated to generate a completely linear
AD SG list.  This unfortunately broke authencesn which expects
the AD to be divided into at least three parts.

This patch fixes it to cope with the new format.  Later we will
fix it properly to accept arbitrary input and not rely on the
input being linear as part of the AEAD conversion.

Fixes: 7021b2e1 ("esp4: Switch to new AEAD interface")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>