libbpf: add ifindex to enable offload support

BPF programs currently can only be offloaded using iproute2. This
patch will allow programs to be offloaded using libbpf calls.

Signed-off-by: David Beckett <david.beckett@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>