tools: libbpf: recognize offload neutral maps

Add helper to libbpf for recognizing maps which should not have
ifindex set when program is loaded.  These maps only contain
host metadata and therefore are not marked for offload, e.g.
the perf event map.

Use this helper in bpf_prog_load_xattr().

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>