tun: remove unnecessary memory barrier

[ Upstream commit ecef67cb10db7b83b3b71c61dbb29aa070ab0112 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>