libbpf, xsk: Select AF_XDP BPF program based on kernel version

libbpf, xsk: Select AF_XDP BPF program based on kernel version

Add detection for kernel version, and adapt the BPF program based on
kernel support. This way, users will get the best possible performance
from the BPF program.

Signed-off-by: Björn Töpel <[email protected]>
Signed-off-by: Marek Majtyka  <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Maciej Fijalkowski <[email protected]>
Acked-by: Maciej Fijalkowski <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]