app/testpmd: fix MTU after device configure

app/testpmd: fix MTU after device configure

In 'rte_eth_dev_configure()', if 'DEV_RX_OFFLOAD_JUMBO_FRAME' is not set
the max frame size is limited to 'RTE_ETHER_MAX_LEN' (1518).
This is mistake because for the PMDs that has frame size bigger than
"RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN" (18 bytes), the MTU becomes
less than 1500, causing a valid frame with 1500 bytes payload to be
dropped.

Since 'rte_eth_dev_set_mtu()' works as expected, it is called after
'rte_eth_dev_configure()' to fix the MTU.
It may look redundant to set MTU after 'rte_eth_dev_configure()', both
with default values, but it is not, the resulting MTU config can be
different in the device based on frame overhead of the PMD.

And instead of setting the MTU to default value, it is first get via
'rte_eth_dev_get_mtu()' and set again, this is to cover cases MTU
changed from testpmd command line.

'rte_eth_dev_set_mtu()', '-ENOTSUP' error is ignored to prevent
irrelevant warning messages for the virtual PMDs.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Tested-by: Igor Romanov <igor.romanov@oktetlabs.ru>