[ofw] typical values for ci_umv_buf_t

[Sean Hefty]

>I don't think there's a requirement that it be less than a page.  There's code
>in the kernel to copy the input/output buffer to a temporary, non-paged-pool
>buffer allocated in the IOCTL handler at the beginning of IOCTL processing, and
>copied back out when the IOCTL completes.

There's no requirement now, but the IOCTL's are all defined using BUFFERED mode.
If the real usage of the buffers are small, I believe that we can be more
efficient in how the umv data is transferred between userspace and the kernel.
The requirement simply becomes a documentation issue that's already met by the
one library that we have...

- Sean