[NET_DMA]: remove unused dma_memcpy_to_kernel_iovec

Al Viro pointed out that dma_memcpy_to_kernel_iovec() really was
unreachable and thus unused.  The code originally was there to support
in-kernel dma needs, but since it remains unused, we'll pull it out.

Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>