xfrm: Always zero high-order sequence number bits

As we're now always including the high bits of the sequence number
in the IV generation process we need to ensure that they don't
contain crap.

This patch ensures that the high sequence bits are always zeroed
so that we don't leak random data into the IV.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>