This generator generates an IV based on a sequence number by xoring it
with a salt and then encrypting it with the same key as used to encrypt
the plain text.  This algorithm requires that the block size be equal
to the IV size.  It is mainly useful for CBC.

It has one noteworthy property that for IPsec the IV happens to lie
just before the plain text so the IV generation simply increases the
number of encrypted blocks by one.  Therefore the cost of this generator
is entirely dependent on the speed of the underlying cipher.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>