libertas if_usb: Fix crash on 64-bit machines

On a 64-bit kernel, skb->tail is an offset, not a pointer. The libertas
usb driver passes it to usb_fill_bulk_urb() anyway, causing interesting
crashes. Fix that by using skb->data instead.

This highlights a problem with usb_fill_bulk_urb(). It doesn't notice
when dma_map_single() fails and return the error to its caller as it
should. In fact it _can't_ currently return the error, since it returns
void.

So this problem was showing up only at unmap time, after we'd already
suffered memory corruption by doing DMA to a bogus address.

Signed-off-by: David Woodhouse <[e-mail]>
Cc: [e-mail]
Acked-by: David S. Miller <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/libertas/if_usb.c

···
 	/* Fill the receive configuration URB and initialise the Rx call back */
 	usb_fill_bulk_urb(cardp->rx_urb, cardp->udev,
 			  usb_rcvbulkpipe(cardp->udev, cardp->ep_in),
-			  (void *) (skb->tail + (size_t) IPFIELD_ALIGN_OFFSET),
+			  skb->data + IPFIELD_ALIGN_OFFSET,
 			  MRVDRV_ETH_RX_PACKET_BUFFER_SIZE, callbackfn,
 			  cardp);