[IPV6]: Only set nfheader_len for top xfrm dst

We only need to set nfheader_len in the top xfrm dst.  This is because
we only ever read the nfheader_len from the top xfrm dst.

It is also easier to count nfheader_len as part of header_len which
then lets us remove the ugly wrapper functions for incrementing and
decrementing header lengths in xfrm6_policy.c.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>