selftests: net: csum: Fix checksums for packets with non-zero padding

Padding is not included in UDP and TCP checksums. Therefore, reduce the
length of the checksummed data to include only the data in the IP
payload. This fixes spurious reported checksum failures like

rx: pkt: sport=33000 len=26 csum=0xc850 verify=0xf9fe
pkt: bad csum

Technically it is possible for there to be trailing bytes after the UDP
data but before the Ethernet padding (e.g. if sizeof(ip) + sizeof(udp) +
udp.len < ip.len). However, we don't generate such packets.

Fixes: 91a7de85 ("selftests/net: add csum offload test")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240906210743.627413-1-sean.anderson@linux.dev


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