Merge branch 'xfrm: policy: replace session decode with flow dissector'

Florian Westphal says:

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Remove the ipv4+ipv6 session decode functions and use generic flow
dissector to populate the flowi for the policy lookup.

Changes since v2:
- first patch broke CONFIG_XFRM=n builds

Changes since v1:
- Can't use skb_flow_dissect(), we might see skbs that have neither
  skb->sk nor skb->dev set. Flow dissector WARN()s in this case, it
  tries to check for a bpf program assigned in that net namespace.

Add a preparation patch to pass down 'struct net' in
xfrm_decode_session so its available for use in patch 3.

Changes since RFC:

- Drop mobility header support.  I don't think that anyone uses
  this.  MOBIKE doesn't appear to need this either.
- Drop fl6->flowlabel assignment, original code leaves it as 0.

There is no reason for this change other than to remove code.
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Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>