dpdk: fix rte_eth_dev_set_mtu callsites to use same mtu value

dpdk: fix rte_eth_dev_set_mtu callsites to use same mtu value

During dpdk_lib_init, it calculates MRU and MTU and later calls
rte_eth_dev_set_mtu with calculated MTU value. However, dpdk_device_setup
calls rte_eth_dev_set_mtu with hi->max_packet_bytes, which is set to be
MRU value in dpdk_lib_init earlier.

Most of the time, MRU != MTU in dpdk_lib_init and it looks like
hi->max_packet_bytes is treated as MTU in other parts of vpp codebase.
Therefore, dpdk_lib_init should be consistent and use MTU instead of MRU
for hi->max_packet_bytes.

Change-Id: I23ff2a6cd45d6bc819b6f64d5f0fc0490b8a44de
Signed-off-by: Rui Cai <rucai@microsoft.com>