geneve: update skb dst pmtu on tx path

geneve: update skb dst pmtu on tx path

Commit 7be0638ed5db ("vxlan: update skb dst pmtu on tx path") has fixed
a performance issue caused by the change of lower dev's mtu for vxlan.

The same thing needs to be done for geneve as well.

Note that geneve cannot adjust it's mtu according to lower dev's mtu
when creating it. The performance is very low later when netperfing
over it without fixing the mtu manually. This patch could also avoid
this issue.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>