selftests/net: skip partial checksum packets in csum test

Detect packets with ip_summed CHECKSUM_PARTIAL and skip these. These
should not exist, as the test sends individual packets between two
hosts. But if (HW) GRO is on, with randomized content sometimes
subsequent packets can be coalesced.

In this case the GSO packet checksum is converted to a pseudo checksum
in anticipation of sending out as TSO/USO. So the field will not match
the expected value.

Do not count these as test errors.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240501193156.3627344-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>