libbpf: Stop using deprecated bpf_map__is_offload_neutral()

Open-code bpf_map__is_offload_neutral() logic in one place in
to-be-deprecated bpf_prog_load_xattr2.

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/20220202225916.3313522-2-andrii@kernel.org