samples: bpf: Add BPF support for redirect tracepoint

This adds the shared BPF file that will be used going forward for
sharing tracepoint programs among XDP redirect samples.

Since vmlinux.h conflicts with tools/include for READ_ONCE/WRITE_ONCE
and ARRAY_SIZE, they are copied in to xdp_sample.bpf.h along with other
helpers that will be required.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-5-memxor@gmail.com