vnet: l2-classify: prefetch (n+2, n+3) rather than (n+1, n+2) inside dual loop code

vnet: l2-classify: prefetch (n+2, n+3) rather than (n+1, n+2) inside dual loop code

The dual loop within the L2 classifier processes the (n, n+1) packets,
the prefetching was trying to prefetch (n+1, n+2) - thus half of the
prefetches were not used - because the next iteration needs (n+2, n+3).

Change-Id: I827d20845dbdd0dcdcf463ee25661a7921428992
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>