selftests/bpf: Add test for open coded dmabuf_iter

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>