net/ice: fix outer UDP Tx checksum offload

net/ice: fix outer UDP Tx checksum offload

If hardware outer UDP Tx checksum offload enabled, it doesn't take
effect when 'IPv6/UDP/VXLAN' packet sent with wrong outer UDP checksum.

In order to take effect, set the 'L4T_CS' flag valid only when 'L4TUNT'
equals one and 'EIPT' is not zero. If 'L4T_CS' flag marked, the hardware
can calculate the outer tunneling UDP checksum.

Fixes: bd70c451532c ("net/ice: support Tx checksum offload for tunnel")
Cc: stable@dpdk.org
Signed-off-by: Murphy Yang <murphyx.yang@intel.com>
Tested-by: Wei Xie <weix.xie@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>