selftests/bpf: Test that exclusive maps are rejected as iter targets

selftests/bpf: Test that exclusive maps are rejected as iter targets

Add a subtest to map_excl that creates an exclusive map and verifies a
bpf_map_elem iterator cannot be attached to it, which would otherwise
let an unrelated program read and overwrite the map's contents through
the iterator's writable value buffer.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t map_excl
  [...]
  ./test_progs -t map_excl
  [    1.704382] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.706068] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #215/1   map_excl/map_excl_allowed:OK
  #215/2   map_excl/map_excl_denied:OK
  #215/3   map_excl/map_excl_no_map_in_map:OK
  #215/4   map_excl/map_excl_no_map_iter:OK
  #215     map_excl:OK
  Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260602133052.423725-5-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>