selftests: net: Fix checksums in xdp_native

selftests: net: Fix checksums in xdp_native

Data adjustment cases failed with "Data exchange failed" when using IPv4
because the program did not update the IP and UDP checksums in the IPv4
branch. The issue was masked when both IPv4 and IPv6 were configured,
since the test harness prefers IPv6.

While here, generalize csum_fold_helper() to fold twice so it works for
any 32-bit input.

Fixes: 0b65cfcef9c5 ("selftests: drv-net: Test tail-adjustment support")
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20260520153928.3371765-1-noren@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>