Merge branch 'add TCP_BPF_SOCK_OPS_CB_FLAGS to bpf_*sockopt()'

Alan Maguire says:

====================
As previously discussed here [1], long-lived sockets can miss
a chance to set additional callbacks if a sock ops program
was not attached early in their lifetime.  Adding support
to bpf_setsockopt() to set callback flags (and bpf_getsockopt()
to retrieve them) provides other opportunities to enable callbacks,
either directly via a cgroup/setsockopt intercepted setsockopt()
or via a socket iterator.

Patch 1 adds bpf_[get|set]sockopt() support; patch 2 adds testing
for it via a sockops programs, along with verification via a
cgroup/getsockopt program.

Changes since v1 [2]:

- Removed unneeded READ_ONCE() (Martin, patch 1)
- Reworked sockopt test to leave existing tests undisturbed while adding
  test_nonstandard_opt() test to cover the TCP_BPF_SOCK_OPS_CB_FLAGS
  case; test verifies that value set via bpf_setsockopt() is what we
  expect via a call to getsockopt() which is caught by a
  cgroup/getsockopt program to provide the flags value (Martin, patch 2)
- Removed unneeded iterator test (Martin)

[1] https://lore.kernel.org/bpf/f42f157b-6e52-dd4d-3d97-9b86c84c0b00@oracle.com/
[2] https://lore.kernel.org/bpf/20240802152929.2695863-1-alan.maguire@oracle.com/
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>