net: flow_dissector: add __skb_get_poff to get a start offset to payload

__skb_get_poff() returns the offset to the payload as far as it could
be dissected. The main user is currently BPF, so that we can dynamically
truncate packets without needing to push actual payload to the user
space and instead can analyze headers only.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>