geneve: Unregister pernet subsys on module unload.

The pernet ops aren't ever unregistered, which causes a memory
leak and an OOPs if the module is ever reinserted.

Fixes: 0b5e8b8eeae4 ("net: Add Geneve tunneling protocol driver")
CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>