xfrm: Flushing empty SPD generates false events

Observed similar behavior on SPD as previouly seen on SAD flushing..
This fixes it.

cheers,
jamal
commit 428b20432dc31bc2e01a94cd451cf5a2c00d2bf4
Author: Jamal Hadi Salim <hadi@cyberus.ca>
Date:   Thu Feb 11 05:49:38 2010 -0500

    xfrm: Flushing empty SPD generates false events

    To see the effect make sure you have an empty SPD.
    On window1 "ip xfrm mon" and on window2 issue "ip xfrm policy flush"
    You get prompt back in window1 and you see the flush event on window2.
    With this fix, you still get prompt on window1 but no event on window2.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>

Signed-off-by: David S. Miller <davem@davemloft.net>