Function build_prp_list 

unsafe fn build_prp_list(buf_phys: u64, byte_count: usize) -> (u64, u64)

Build PRP (Physical Region Page) list for a physically contiguous buffer.

Returns (prp1, prp2):

• If the buffer fits in a single page: prp1 = buf_phys, prp2 = 0
• If the buffer spans multiple pages: prp1 = buf_phys, prp2 = PRP list phys addr

The caller must ensure buf_phys is page-aligned and the buffer is physically contiguous for multi-page transfers.