gro: reset skb_iif on reuse

Like Herbert's change from a few days ago:

66c46d74 gro: Reset dev pointer on reuse

this may not be necessary at this point, but we should still clean up
the skb->skb_iif.  If not we may end up with an invalid valid for
skb->skb_iif when the skb is reused and the check is done in
__netif_receive_skb.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>