selftests: bpf: add check for ip XDP redirect

Kernel test robot reports error when running test_xdp_redirect.sh.
Check if ip tool supports xdpgeneric, if not, skip the test.

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