netdev-dpdk: use rte_eth_dev_set_mtu.

netdev-dpdk: use rte_eth_dev_set_mtu.

DPDK provides an API to set the MTU of compatible physical devices -
rte_eth_dev_set_mtu(). Prior to DPDK v16.07 however, this API was not
implemented in some DPDK PMDs (i40e, specifically). To allow the use
of jumbo frames with affected NICs in OvS-DPDK, MTU configuration was
achieved by setting the jumbo frame flag, and corresponding maximum
permitted Rx frame size, in an rte_eth_conf structure for the NIC
port, and subsequently invoking rte_eth_dev_configure() with that
configuration.

However, that method does not set the MTU field of the underlying DPDK
structure (rte_eth_dev) for the corresponding physical device;
consequently, rte_eth_dev_get_mtu() reports the incorrect MTU for an
OvS-DPDK phy device with non-standard MTU.

Resolve this issue by invoking rte_eth_dev_set_mtu() when setting up
or modifying the MTU of a DPDK phy port.

Fixes: 0072e93 ("netdev-dpdk: add support for jumbo frames")
Reported-by: Aaron Conole <aconole@redhat.com>
Reported-by: Vipin Varghese <vipin.varghese@intel.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Acked-by: Sugesh Chandran <sugesh.chandran@intel.com>
Tested-by: Sugesh Chandran <sugesh.chandran@intel.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>