selftests/bpf: Add test for truncated dmabuf_iter reads

selftests/bpf: Add test for truncated dmabuf_iter reads

If many dmabufs are present, reads of the dmabuf iterator can be
truncated at PAGE_SIZE or user buffer size boundaries before the fix in
"bpf: Fix truncated dmabuf iterator reads". Add a test to
confirm truncation does not occur.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Link: https://lore.kernel.org/r/20251204000348.1413593-2-tjmercier@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>