examples/l3fwd: share queue size variables

examples/l3fwd: share queue size variables

nb_rxd and nb_txd are used in polling mode and event mode of
operation. nb_rxd and nb_txd are already global in polling mode
but are not visible to event mode code. Make them visible to all
parts of the application.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Kathleen Capella <kathleen.capella@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Yingya Han < yingyax.han@intel.com>