[AH6] Disallow mutable bits after AH header

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

Hi:

As we discussed before, mutable headers should not be allowed after
the AH header.  In fact, this appears to be the intention of RFC 2402.
It is further clarified in section 3.1.1 of

http://www.ietf.org/internet-drafts/draft-ietf-ipsec-rfc2402bis-07.txt

This allows us to simplify the code in ah6.c.  As a result, this also
fixes the following issues:

* Dependence on skb->h in ah6_output().
* Bogus clearing of auth_data of 2nd AH header in ipv6_clear_mutable_options().

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

Cheers,
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Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
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