selftests/xsk: generate data for multi-buffer packets

Add the ability to generate data in the packets that are correct for
multi-buffer packets. The ethernet header should only go into the
first fragment followed by data and the others should only have
data. We also need to modify the pkt_dump function so that it knows
what fragment has an ethernet header so it can print this.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230516103109.3066-10-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>