selftests/bpf: Remove usage of deprecated feature probing APIs

Switch to libbpf_probe_*() APIs instead of the deprecated ones.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220202225916.3313522-5-andrii@kernel.org