netvsc: fix checksum on UDP IPV6

The software calculation of UDP checksum in Netvsc driver was
only handling IPv4 case. By using skb_checksum_help() instead
all protocols can be handled. Rearrange code to eliminate goto
and look like other drivers.

This is a temporary solution; recent versions of Window Server etc
do support UDP checksum offload, just need to do the appropriate negotiation
with host to validate before using. This will be done in later patch.

Please queue this for -stable as well.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>