BACKPORT: bpf: Add open coded dmabuf iterator

This open coded iterator allows for more flexibility when creating BPF
programs. It can support output in formats other than text. With an open
coded iterator, a single BPF program can traverse multiple kernel data
structures (now including dmabufs), allowing for more efficient analysis
of kernel data compared to multiple reads from procfs, sysfs, or
multiple traditional BPF iterator invocations.

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/20250522230429.941193-4-tjmercier@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
(cherry picked from commit 6eab7ac7)
[TJ: __bpf_kfunc_{start,end}_defs don't exist]
Change-Id: I95e1eb95daa516858cf69c581edb7f5b1d3637c7
Bug: 421890159
Signed-off-by: T.J. Mercier <tjmercier@google.com>