BACKPORT: 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>
(cherry picked from commit ae5d2c59)
Change-Id: I88ba76aa00ea6c551e843e5ad4e3018ef54808f9
[TJ: #define _GNU_SOURCE directly]
Bug: 421890159
Bug: 449985230
Test: tools/testing/selftests/bpf/vmtest.sh "./test_progs -t dmabuf_iter/default_iter"
Signed-off-by: T.J. Mercier <tjmercier@google.com>