[XFRM]: Fix aevent timer.

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: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>