bpf: add csum_diff helper to xdp as well

Useful for porting cls_bpf programs w/o increasing program
complexity limits much at the same time, so add the helper
to XDP as well.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>