selftests/bpf: Add test for dmabuf_iter

selftests/bpf: Add test for dmabuf_iter

This test creates a udmabuf, and a dmabuf from the system dmabuf heap,
and uses a BPF program that prints dmabuf metadata with the new
dmabuf_iter to verify they can be found.

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/20250522230429.941193-5-tjmercier@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>