net/netfilter: Add unstable CT lookup helpers for XDP and TC-BPF

This change adds conntrack lookup helpers using the unstable kfunc call
interface for the XDP and TC-BPF hooks. The primary usecase is
implementing a synproxy in XDP, see Maxim's patchset [0].

Export get_net_ns_by_id as nf_conntrack_bpf.c needs to call it.

This object is only built when CONFIG_DEBUG_INFO_BTF_MODULES is enabled.

  [0]: https://lore.kernel.org/bpf/20211019144655.3483197-1-maximmi@nvidia.com



Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220114163953.1455836-7-memxor@gmail.com


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