net: netpoll: Initialize UDP checksum field before checksumming

In the Linux kernel, the following vulnerability has been resolved: net: netpoll: Initialize UDP checksum field before checksumming commit f1fce08e63fe ("netpoll: Eliminate redundant assignment") removed the initialization of the UDP checksum, which was wrong and broke netpoll IPv6 transmission due to bad checksumming. udph->check needs to be set before calling csum_ipv6_magic().