If user spaces calls read(2), the vfs_read() in fs/read_write.c
does not create an iov array any more since v5.19-10288-g3e20a751aff0.

That commit does as follows:

   -       struct iovec iov = { .iov_base = buf, .iov_len = len };
   -       iov_iter_init(&iter, READ, &iov, 1, len);
   +       iov_iter_ubuf(&iter, READ, buf, len);

as a resuly, our code that checks iter->iov->nr_segs sees 0 segments,
understands we have no buffer, and returns 0. Same for write(2).

This is a hack, as I don'w know what happens if the user space uses readv/writev.
Actually, I'd better have a simple fops->read() and fops->write() in the driver,
but I'm sure I lack a lot of background knowledge here.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>