Send aevent immediately if we have sent nothing since last timer and
this is the first packet.

Fixes a corner case when packet threshold is very high, the timer low
and a very low packet rate input which is bursty.
Signed-off-by: NJamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>