vxlan: few locking fixes in nexthop event handler

- remove fdb from nh_list before the rcu grace period
- protect fdb->vdev with rcu
- hold spin lock before destroying fdb

Fixes: c7cdbe2e

 ("vxlan: support for nexthop notifiers")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>