USBNET: support DMA SG

USBNET: support DMA SG

This patch introduces support of DMA SG if the USB host controller
which usbnet device is attached to is capable of building packet from
discontinuous buffers.

The patch supports passing the skb fragment buffers to usb stack directly
via urb->sg.

Cc: Ben Hutchings <[email protected]>
Cc: Grant Grundler <[email protected]>
Cc: Freddy Xin <[email protected]>
Cc: Alan Stern <[email protected]>
Acked-by: Oliver Neukum <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Acked-by: David S. Miller <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>