tun: remove unnecessary memory barrier

Replace set_current_state with __set_current_state since no memory
barrier is needed at this point.

Signed-off-by: Timur Celik <mail@timurcelik.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>