net/null: support bulk allocation

net/null: support bulk allocation

Bulk allocation of multiple mbufs increased more than ~2%  and less
than 8% throughput on single core (1.8 GHz), based on usage for example
1: Testpmd case: Two null devices with copy 8% improvement.
    testpmd -c 0x3 -n 4 --socket-mem 1024,1024
	--vdev 'eth_null0,size=64,copy=1' --vdev 'eth_null1,size=64,copy=1'
	-- -i -a --coremask=0x2 --txrst=64 --txfreet=64 --txd=256
	--rxd=256 --rxfreet=64 --burst=64 --txpt=64 --txq=1 --rxq=1 --numa
2. Ovs switch case: 2% improvement.
$VSCTL add-port ovs-br dpdk1 -- set Interface dpdk1 type=dpdk \
        options:dpdk-devargs=eth_null0,size=64,copy=1
$VSCTL add-port ovs-br dpdk2 -- set Interface dpdk2 type=dpdk \
        options:dpdk-devargs=eth_null1,size=64,copy=1

Signed-off-by: Mallesh Koujalagi <malleshx.koujalagi@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>