libbpf: use explicit map reuse flag to skip map creation steps

Instead of inferring whether map already point to previously
created/pinned BPF map (which user can specify with bpf_map__reuse_fd()) API),
use explicit map->reused flag that is set in such case.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240104013847.3875810-3-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>