app/testpmd: fix max Rx packet length for VLAN packets

app/testpmd: fix max Rx packet length for VLAN packets

When the max rx packet length is smaller than the sum of mtu size and
ether overhead size, it should be enlarged, otherwise the VLAN packets
will be dropped.

Removed the rx_offloads assignment for jumbo frame during command line
parsing, and set the correct jumbo frame flag if MTU size is larger than
the default value 'RTE_ETHER_MTU' within 'init_config()'.

Fixes: 384161e00627 ("app/testpmd: adjust on the fly VLAN configuration")
Fixes: 35b2d13fd6fd ("net: add rte prefix to ether defines")
Fixes: ce17eddefc20 ("ethdev: introduce Rx queue offloads API")
Fixes: 150c9ac2df13 ("app/testpmd: update Rx offload after setting MTU")
Cc: stable@dpdk.org
Signed-off-by: Steve Yang <stevex.yang@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>