CVE-2026-74360: bpf: Reject exclusive maps for bpf_map_elem iterators

In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject exclusive maps for bpfmapelem iterators

Exclusive maps (aka exclproghash) are meant to be reachable only from the single program whose hash matches. This is enforced by checkmapprogcompatibility() when the map is referenced from a program such as signed BPF loaders.

A bpfmapelem iterator, however, binds its target map at attach time in bpfiterattachmap() 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.