selftests/bpf: Consistent malloc/calloc usage

Use calloc instead of malloc where it makes sense, and avoid C++-style
void *-cast.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210122154725.22140-12-bjorn.topel@gmail.com