tun: avoid extra timer schedule in tun_flow_cleanup()

If tun_flow_cleanup() deleted all flows, no need to
arm the timer again. It will be armed next time
tun_flow_update() is called.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>