bpf: Refactor srcID resolution on bpf_netdev egress
bpf: Refactor srcID resolution on bpf_netdev egress

Regroup all identity resolution code for egress (finalize_sec_ctx(),
derive_ipv4_sec_ctx(), ipcache lookup, and FROM_HOST-deduced) in a single
function and remove unused code. Also rename identity variables to
clarify origin.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

tgraf/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium