libbpf: Reject static maps

Static maps never really worked with libbpf, because all such maps were always
silently resolved to the very first map. Detect static maps (both legacy and
BTF-defined) and report user-friendly error.

Tested locally by switching few maps (legacy and BTF-defined) in selftests to
static ones and verifying that now libbpf rejects them loudly.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210513233643.194711-2-andrii@kernel.org