net: netpoll: Initialize UDP checksum field before checksumming

commit f1fce08e ("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().

Fixes: f1fce08e ("netpoll: Eliminate redundant assignment")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250620-netpoll_fix-v1-1-f9f0b82bc059@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>