selftests/bpf: Stop using bpf_map__def() API

libbpf bpf_map__def() API is being deprecated, replace selftests/bpf's
usage with the appropriate getters and setters.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220108004218.355761-5-christylee@fb.com