Commit 49974420 (tuntap: dont use skb after netif_rx_ni(skb))
introduced another bug.

skb_get_rxhash() needs to access the network header, and it was
set for us in netif_rx_ni().

We need to reset network header or else skb_flow_dissect() behavior
is out of control.
Reported-and-tested-by: NKirill A. Shutemov <kirill@shutemov.name>
Tested-by: NDaniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>