net/ixgbe: fix jumbo frame flag condition

net/ixgbe: fix jumbo frame flag condition

The jumbo frame uses the 'RTE_ETHER_MAX_LEN' as boundary condition.
If the Ether overhead is larger than 18 when it supports VLAN tag,
that will cause the jumbo flag rx offload is wrong when MTU size is
'RTE_ETHER_MTU'.

This fix will normalize the boundary condition with 'RTE_ETHER_MTU'
and overhead even though current overhead is 18.

Fixes: 59d0ecdbf0e1 ("ethdev: MTU accessors")
Fixes: 95a27b3ba5f5 ("net/ixgbe: enable jumbo frame for VF")
Cc: stable@dpdk.org
Signed-off-by: Steve Yang <stevex.yang@intel.com>
Acked-by: Jeff Guo <jia.guo@intel.com>