udp: fixup csum for GSO receive slow path

When UDP packets generated locally by a socket with UDP_SEGMENT
traverse the following path:

UDP tunnel(xmit) -> veth (segmentation) -> veth (gro) ->
	UDP tunnel (rx) -> UDP socket (no UDP_GRO)

ip_summed will be set to CHECKSUM_PARTIAL at creation time and
such checksum mode will be preserved in the above path up to the
UDP tunnel receive code where we have:

 __iptunnel_pull_header() -> skb_pull_rcsum() ->
skb_postpull_rcsum() -> __skb_postpull_rcsum()

The latter will convert the skb to CHECKSUM_NONE.

The UDP GSO packet will be later segmented as part of the rx socket
receive operation, and will present a CHECKSUM_NONE after segmentation.

Additionally the segmented packets UDP CB still refers to the original
GSO packet len. Overall that causes unexpected/wrong csum validation
errors later in the UDP receive path.

We could possibly address the issue with some additional checks and
csum mangling in the UDP tunnel code. Since the issue affects only
this UDP receive slow path, let's set a suitable csum status there.

Note that SKB_GSO_UDP_L4 or SKB_GSO_FRAGLIST packets lacking an UDP
encapsulation present a valid checksum when landing to udp_queue_rcv_skb(),
as the UDP checksum has been validated by the GRO engine.

v2 -> v3:
 - even more verbose commit message and comments

v1 -> v2:
 - restrict the csum update to the packets strictly needing them
 - hopefully clarify the commit message and code comments

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>