Merge branch 'bpf: tcp: Allow bpf-tcp-cc to call bpf_(get|set)sockopt'

Martin KaFai says:

====================

This set allows the bpf-tcp-cc to call bpf_setsockopt.  One use
case is to allow a bpf-tcp-cc switching to another cc during init().
For example, when the tcp flow is not ecn ready, the bpf_dctcp
can switch to another cc by calling setsockopt(TCP_CONGESTION).

bpf_getsockopt() is also added to have a symmetrical API, so
less usage surprise.

v2:
- Not allow switching to kernel's tcp_cdg because it is the only
  kernel tcp-cc that stores a pointer to icsk_ca_priv.
  Please see the commit log in patch 1 for details.
  Test is added in patch 4 to check switching to tcp_cdg.
- Refactor the logic finding the offset of a func ptr
  in the "struct tcp_congestion_ops" to prog_ops_moff()
  in patch 1.
- bpf_setsockopt() has been disabled in release() since v1 (please
  see commit log in patch 1 for reason).  bpf_getsockopt() is
  also disabled together in release() in v2 to avoid usage surprise
  because both of them are usually expected to be available together.
  bpf-tcp-cc can already use PTR_TO_BTF_ID to read from tcp_sock.
====================

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