selftests/bpf: Fix dmabuf_iter/lots_of_buffers failure with 64K page

selftests/bpf: Fix dmabuf_iter/lots_of_buffers failure with 64K page

On arm64 with 64K page , I observed the following test failure:
  ...
  subtest_dmabuf_iter_check_lots_of_buffers:FAIL:total_bytes_read unexpected total_bytes_read:
      actual 4696 <= expected 65536
  #97/3    dmabuf_iter/lots_of_buffers:FAIL

With 4K page on x86, the total_bytes_read is 4593.
With 64K page on arm64, the total_byte_read is 4696.

In progs/dmabuf_iter.c, for each iteration, the output is
  BPF_SEQ_PRINTF(seq, "%lu\n%llu\n%s\n%s\n", inode, size, name, exporter);

The only difference between 4K and 64K page is 'size' in
the above BPF_SEQ_PRINTF. The 4K page will output '4096' and
the 64K page will output '65536'. So the total_bytes_read with 64K page
is slighter greater than 4K page.

Adjusting the total_bytes_read from 65536 to 4096 fixed the issue.

Cc: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260113061023.3798085-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>