bpf/flow_dissector: document flags

  * ``nhoff`` - initial offset of the networking header

  * ``thoff`` - initial offset of the transport header, initialized to nhoff

  * ``n_proto`` - L3 protocol type, parsed out of L2 header

  * ``flags`` - optional flags

Flow dissector BPF program should fill out the rest of the ``struct

bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be

handle both cases.

Flags

=====

``flow_keys->flags`` might contain optional input flags that work as follows:

* ``BPF_FLOW_DISSECTOR_F_PARSE_1ST_FRAG`` - tells BPF flow dissector to

  continue parsing first fragment; the default expected behavior is that

  flow dissector returns as soon as it finds out that the packet is fragmented;

  used by ``eth_get_headlen`` to estimate length of all headers for GRO.

* ``BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL`` - tells BPF flow dissector to

  stop parsing as soon as it reaches IPv6 flow label; used by

  ``___skb_get_hash`` and ``__skb_get_hash_symmetric`` to get flow hash.

* ``BPF_FLOW_DISSECTOR_F_STOP_AT_ENCAP`` - tells BPF flow dissector to stop

  parsing as soon as it reaches encapsulated headers; used by routing

  infrastructure.

Reference Implementation

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