bpf: rework lb{4,6}_select_backend_slot() to return backend id
bpf: rework lb{4,6}_select_backend_slot() to return backend id

This reduces one lookup in case of Maglev and simplifies backend management
in Maglev maps given we only need to populate backend ids there instead of
slot id indirection. The slot ids are still used by socket LB when Maglev is
active. For externalTrafficPolicy=Local, we don't need slot id entries in
svc map for external scope though.

Signed-off-by: Daniel Borkmann <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@iogearbox.net>
Signed-off-by: Martynas Pumputis <6b0d31c0d563223024da45691584643ac78c96e8@lambda.lt>

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