[c-nsp] high CPU with ERSPAN on sup720

[Mark Boolootian]

> I'm trying to turn up an ERSPAN session on a sup720 w/PFC3BXL, and I'm
> finding that my CPU is going from essentially idle to fully utilized
> when being fed by two ports.

I might add that the two source ports are on an WS-X6516-GBIC and
the destination port for the traffic is on the sup720.