samples: bpf: Convert xdp_redirect_kern.o to XDP samples helper

We moved swap_src_dst_mac to xdp_sample.bpf.h to be shared with other
potential users, so drop it while moving code to the new file.
Also, consistently use SEC("xdp") naming instead.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-16-memxor@gmail.com