ovs/gre,geneve: fix error path when creating an iface

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: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>