examples/l2fwd: increase pktmbuf pool size
examples/l2fwd: increase pktmbuf pool size

Make pktmbuf pool size a function of ports and lcores detected instead
of using constant 8192.

Signed-off-by: Pavan Nikhilesh <8184a94f8f7778086ea0697cdb513b2e42813b09@caviumnetworks.com>
Acked-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk