vxlan: use a more suitable function when assigning NULL

When stopping the vxlan interface we detach it from the socket.
Use RCU_INIT_POINTER() and not rcu_assign_pointer() to do so.

Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>