net: netfilter: add bpf_ct_set_nat_info kfunc helper

Introduce bpf_ct_set_nat_info kfunc helper in order to set source and
destination nat addresses/ports in a new allocated ct entry not inserted
in the connection tracking table yet.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/9567db2fdfa5bebe7b7cc5870f7a34549418b4fc.1663778601.git.lorenzo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>