Merge branch 'bpf_tcp_check_syncookie'

Lorenz Bauer says:

====================
This series adds the necessary helpers to determine wheter a given
(encapsulated) TCP packet belongs to a connection known to the network stack.

* bpf_skc_lookup_tcp gives access to request and timewait sockets
* bpf_tcp_check_syncookie identifies the final 3WHS ACK when syncookies
  are enabled

The goal is to be able to implement load-balancing approaches like
glb-director [1] or Beamer [2] in pure eBPF. Specifically, we'd like to replace
the functionality of the glb-redirect kernel module [3] by an XDP program or
tc classifier.

Changes in v3:
* Fix missing check for ip4->ihl
* Only cast to unsigned long in BPF_CALLs

Changes in v2:
* Rename bpf_sk_check_syncookie to bpf_tcp_check_syncookie.
* Add bpf_skc_lookup_tcp. Without it bpf_tcp_check_syncookie doesn't make sense.
* Check tcp_synq_no_recent_overflow() in bpf_tcp_check_syncookie.
* Check th->syn in bpf_tcp_check_syncookie.
* Require CONFIG_IPV6 to be a built in.

1: https://github.com/github/glb-director
2: https://www.usenix.org/conference/nsdi18/presentation/olteanu
3: https://github.com/github/glb-director/tree/master/src/glb-redirect
====================

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