Revert "netfilter: conntrack: mark UDP zero checksum as CHECKSUM_UNNECESSARY"

This reverts commit 5bed9f3f.

Gal Presman says:
 this patch broke geneve tunnels, or possibly all udp tunnels?
 A simple test that creates two geneve tunnels and runs tcp iperf fails
 and results in checksum errors (TcpInCsumErrors).

Original commit wanted to fix nf_reject with zero checksum,
so it appears better to change nf reject infra instead.

Fixes: 5bed9f3f

 ("netfilter: conntrack: mark UDP zero checksum as CHECKSUM_UNNECESSARY")
Reported-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>