After ipgre_newlink()/geneve_configure() call, the netdev is registered.

Fixes: 7e059158 ("vxlan, gre, geneve: Set a large MTU on ovs-created tunnel devices")
CC: David Wragg <david@weave.works>
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>