samples/bpf: fixup some tools to be able to support xdp multibuffer

This changes the section name for the bpf program embedded in these
files to "xdp.frags" to allow the programs to be loaded on drivers that
are using an MTU greater than PAGE_SIZE.  Rather than directly accessing
the buffers, the packet data is now accessed via xdp helper functions to
provide an example for those who may need to write more complex
programs.

v2: remove new unnecessary variable

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/20220621175402.35327-1-gospo@broadcom.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>