selftests/bpf: Change type from void * to struct generic_data *

Instead of casting from void *, let us use the actual type in
gen_udp_hdr().

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210122154725.22140-9-bjorn.topel@gmail.com