dpdk: Fix MTU calc for NICs that support mtu<9216

dpdk: Fix MTU calc for NICs that support mtu<9216

Problem: rte_eth_dev_set_mtu() returns with failure from ThunderX NICVF
DPDK PMD driver which supports MTU less than ETHERNET_MAX_PACKET_BYTES.
rte_eth_dev_set_mtu() being called twice from dpdk_lib_init(): one via
dpdk_device_setup() and second in dpdk_lib_init() itself. Currently
dpdk_lib_init() passes vnet_hardware_interface->max_packet_bytes as an
argument to rte_eth_dev_set_mtu() without consulting dev_info.max_rx_pktlen.
NICs like i4oe, ixgbe can support MTU much greater than 9216 hence its not
a problem for those NICS.

Fix: This patch calculates dpdk_device->port_conf.rxmode.max_rx_pkt_len,
vnet_hardware_interface->max_packet_bytes and MTU by consulting
dev_info.max_rx_pktlen.

Change-Id: If04bbfae49ee971dac0063ff1835e4a9c3087865
Signed-off-by: Nitin Saxena <nitin.saxena@cavium.com>