bpf: Copy map value to stack on <4.18 kernels
bpf: Copy map value to stack on <4.18 kernels

Before Linux commit d71962f3e627 (4.18), map helpers were not allowed to
access map values directly. So for those older kernels, we need to copy
the map value from the tail call buffer to the stack first. We don't have
a probe for that, but the bpf_fib_lookup helper was introduced in the
same Linux release. So let's use that helper until we have a proper
probe.

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

michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium