bpf: Reject exclusive maps for bpf_map_elem iterators

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject exclusive maps for bpf_map_elem iterators Exclusive maps (aka excl_prog_hash) are meant to be reachable only from the single program whose hash matches. This is enforced by check_map_prog_compatibility() when the map is referenced from a program such as signed BPF loaders. A bpf_map_elem iterator, however, binds its target map at attach time in bpf_iter_attach_map() instead of referencing it from the program, so the exclusivity check is never reached. On top of that, the iterator exposes the map value as a writable buffer.