Regression due to /sys/kernel/dmabuf/buffers removal

[Julian Orth]

In ab4c3dcf9a71582503b4fb25aeab884c696cab25 ("dma-buf: Remove DMA-BUF
sysfs stats") the /sys/kernel/dmabuf/buffer directory was removed.

I've been using this interface, specifically the exporter_name file,
to detect dmabufs created via udmabuf. Such dmabufs show "udmabuf" in
exporter_name. I've been doing this for two reasons: 1) to detect that
mmap on such buffers will be fast and 2) to detect that GPU access to
such buffers will be slow.

With the removal of that file, that detection mechanism no longer works.

I'm not particularly fond of that mechanism but it was the only one
providing that functionality that I could find at the time. If there
is another one, ideally an ioctl on the dmabuf, please let me know.

Shipping an entire BPF compiler in my application, which the original
patch suggests as the replacement, is not an option when the removed
alternative was simply reading a file.

Thanks, Julian