crypto: mask the crypto operations from totem packet size management

totem doesn't need to understand what crypto does.

totem needs to be able to tell crypto: "those are data, play with them"
and crypto needs to return: "here are your scrambled data and the new size"

similar to decrypt/verify.

this way we add enough dynamic within crypto to change header size and all
at any given time (for different hash algorithm for example) without
affecting on wire compat.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>