selftests: Calculate udpgso segment count without header adjustment

selftests: Calculate udpgso segment count without header adjustment

The below referenced commit correctly updated the computation of number
of segments (gso_size) by using only the gso payload size and
removing the header lengths.

With this change the regression test started failing. Update
the tests to match this new behavior.

Both IPv4 and IPv6 tests are updated, as a separate patch in this series
will update udp_v6_send_skb to match this change in udp_send_skb.

Fixes: f3de503872c7 ("udp: using datalen to cap max gso segments")
Signed-off-by: Coco Li <lixiaoyan@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20211223222441.2975883-2-lixiaoyan@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>