Merge branch 'flow_dissector-input-flags'

Stanislav Fomichev says:

====================
C flow dissector supports input flags that tell it to customize parsing
by either stopping early or trying to parse as deep as possible.
BPF flow dissector always parses as deep as possible which is sub-optimal.
Pass input flags to the BPF flow dissector as well so it can make the same
decisions.

Series outline:
* remove unused FLOW_DISSECTOR_F_STOP_AT_L3 flag
* export FLOW_DISSECTOR_F_XXX flags as uapi and pass them to BPF
  flow dissector
* add documentation for the export flags
* support input flags in BPF_PROG_TEST_RUN via ctx_{in,out}
* sync uapi to tools
* support FLOW_DISSECTOR_F_PARSE_1ST_FRAG in selftest
* support FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL in kernel and selftest
* support FLOW_DISSECTOR_F_STOP_AT_ENCAP in selftest

Pros:
* makes BPF flow dissector faster by avoiding burning extra cycles
* existing BPF progs continue to work by ignoring the flags and always
  parsing as deep as possible

Cons:
* new UAPI which we need to support (OTOH, if we need to deprecate some
  flags, we can just stop setting them upon calling BPF programs)

Some numbers (with .repeat = 4000000 in test_flow_dissector):
        test_flow_dissector:PASS:ipv4-frag 35 nsec
        test_flow_dissector:PASS:ipv4-frag 35 nsec
        test_flow_dissector:PASS:ipv4-no-frag 32 nsec
        test_flow_dissector:PASS:ipv4-no-frag 32 nsec

        test_flow_dissector:PASS:ipv6-frag 39 nsec
        test_flow_dissector:PASS:ipv6-frag 39 nsec
        test_flow_dissector:PASS:ipv6-no-frag 36 nsec
        test_flow_dissector:PASS:ipv6-no-frag 36 nsec

        test_flow_dissector:PASS:ipv6-flow-label 36 nsec
        test_flow_dissector:PASS:ipv6-flow-label 36 nsec
        test_flow_dissector:PASS:ipv6-no-flow-label 33 nsec
        test_flow_dissector:PASS:ipv6-no-flow-label 33 nsec

        test_flow_dissector:PASS:ipip-encap 38 nsec
        test_flow_dissector:PASS:ipip-encap 38 nsec
        test_flow_dissector:PASS:ipip-no-encap 32 nsec
        test_flow_dissector:PASS:ipip-no-encap 32 nsec

The improvement is around 10%, but it's in a tight cache-hot
BPF_PROG_TEST_RUN loop.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>