selftests/bpf: Convert legacy prog load APIs to bpf_prog_load()

Convert all the uses of legacy low-level BPF program loading APIs
(mostly bpf_load_program_xattr(), but also some bpf_verify_program()) to
bpf_prog_load() uses.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-10-andrii@kernel.org