crypto: seqiv - Ensure that IV size is at least 8 bytes

Since seqiv is designed for IPsec we need to be able to accomodate
the whole IPsec sequence number in order to ensure the uniqueness
of the IV.

This patch forbids any algorithm with an IV size of less than 8
from using it.  This should have no impact on existing users since
they all have an IV size of 8.

Reported-by: Maciej ?enczykowski <zenczykowski@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Maciej ?enczykowski <zenczykowski@gmail.com>