netfilter: nf_tables: set pktinfo->thoff at AH header if found

Phil Sutter reports that IPv6 AH header matching is broken. From
userspace, nft generates bytecode that expects to find the AH header at
NFT_PAYLOAD_TRANSPORT_HEADER both for IPv4 and IPv6. However,
pktinfo->thoff is set to the inner header after the AH header in IPv6,
while in IPv4 pktinfo->thoff points to the AH header indeed. This
behaviour is inconsistent. This patch fixes this problem by updating
ipv6_find_hdr() to get the IP6_FH_F_AUTH flag so this function stops at
the AH header, so both IPv4 and IPv6 pktinfo->thoff point to the AH
header.

This is also inconsistent when trying to match encapsulated headers:

1) A packet that looks like IPv4 + AH + TCP dport 22 will *not* match.
2) A packet that looks like IPv6 + AH + TCP dport 22 will match.

Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>