BACKPORT: selftests/bpf: Add test for open coded dmabuf_iter

Use the same test buffers as the traditional iterator and a new BPF map
to verify the test buffers can be found with the open coded dmabuf
iterator.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250522230429.941193-6-tjmercier@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
(cherry picked from commit 7594dcb7)
Change-Id: Ia2e79f5f4dd77d3f50251dc28cf36f70f0096bf0
[TJ: BPF verifier doesn't yet support non-constant starting value for bpf_for]
[TJ: #include bpf_experimental.h directly]
Bug: 421890159
Test: tools/testing/selftests/bpf/vmtest.sh "./test_progs -t dmabuf_iter/open_coded"
Signed-off-by: T.J. Mercier <tjmercier@google.com>