net: hns3: Fix for geneve tx checksum bug

when skb->encapsulation is 0, skb->ip_summed is CHECKSUM_PARTIAL
and it is udp packet, which has a dest port as the IANA assigned.
the hardware is expected to do the checksum offload, but the
hardware will not do the checksum offload when udp dest port is
6081.

This patch fixes it by doing the checksum in software.

Reported-by: Li Bing <libing@winhong.com>
Signed-off-by: Yi Li <yili@winhong.com>
Signed-off-by: David S. Miller <davem@davemloft.net>