selftests: bpf: add a test for XDP redirect

Add test for xdp_redirect by creating two namespaces with two
veth peers, then forward packets in-between.

Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>