libbpf: Remove internal use of deprecated bpf_prog_load() variants

Remove all the internal uses of bpf_load_program_xattr(), which is
slated for deprecation in v0.7.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-5-andrii@kernel.org