esp6: Use high-order sequence number bits for IV generation

I noticed we were only using the low-order bits for IV generation
when ESN is enabled.  This is very bad because it means that the
IV can repeat.  We must use the full 64 bits.

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