The chain IV generator is the one we've been using in the IPsec stack.
It simply starts out with a random IV, then uses the last block of each
encrypted packet's cipher text as the IV for the next packet.

It can only be used by synchronous ciphers since we have to make sure
that we don't start the encryption of the next packet until the last
one has completed.

It does have the advantage of using very little CPU time since it doesn't
have to generate anything at all.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>