selftests/bpf: Improve readability of xdpxceiver/worker_pkt_validate()

Introduce a local variable to get rid of lot of casting. Move common
code outside the if/else-clause.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210122154725.22140-6-bjorn.topel@gmail.com