netfilter: nat: never update the UDP checksum when it's 0

If the UDP header of a local VXLAN endpoint is NAT-ed, and the VXLAN
device has disabled UDP checksums and enabled Tx checksum offloading,
then the skb passed to udp_manip_pkt() has hdr->check == 0 (outer
checksum disabled) and skb->ip_summed == CHECKSUM_PARTIAL (inner packet
checksum offloaded).

Because of the ->ip_summed value, udp_manip_pkt() tries to update the
outer checksum with the new address and port, leading to an invalid
checksum sent on the wire, as the original null checksum obviously
didn't take the old address and port into account.

So, we can't take ->ip_summed into account in udp_manip_pkt(), as it
might not refer to the checksum we're acting on. Instead, we can base
the decision to update the UDP checksum entirely on the value of
hdr->check, because it's null if and only if checksum is disabled:

  * A fully computed checksum can't be 0, since a 0 checksum is
    represented by the CSUM_MANGLED_0 value instead.

  * A partial checksum can't be 0, since the pseudo-header always adds
    at least one non-zero value (the UDP protocol type 0x11) and adding
    more values to the sum can't make it wrap to 0 as the carry is then
    added to the wrapped number.

  * A disabled checksum uses the special value 0.

The problem seems to be there from day one, although it was probably
not visible before UDP tunnels were implemented.

Fixes: 5b1158e9 ("[NETFILTER]: Add NAT support for nf_conntrack")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>