net: Make USO depend on CSUM offload

UDP segmentation offload inherently depends on checksum offload. It should
not be possible to disable checksum offload while leaving USO enabled.
Enforce this dependency in code.

There is a single tx-udp-segmentation feature flag to indicate support for
both IPv4/6, hence the devices wishing to support USO must offer checksum
offload for both IP versions.

Fixes: 10154dbd ("udp: Allow GSO transmit from devices with no checksum offload")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240808-udp-gso-egress-from-tunnel-v4-1-f5c5b4149ab9@cloudflare.com


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