tuntap: reset network header before calling skb_get_rxhash()

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: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>