generic l2tp: drop flow hash on forward

generic l2tp: drop flow hash on forward

Drop the flow-hash of the skb when forwarding to the L2TP netdev.

This avoids the L2TP qdisc from using the flow-hash from the outer
packet, which is identical for every flow within the tunnel.

This does not affect every platform but is specific for the ethernet
driver. It depends on the platform including L4 information in the
flow-hash.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 35a5e62da7275a4a1423df6238340dd08eeff3c9)