tun: Turn tun_flow_init() into void fn

This routine doesn't fail since 9fdc6bef

 (tuntap: dont use a private kmem_cache)
so it makes sense to compact the code a little bit.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>