udp: gso: Use single MSS length in UDP header for GSO_PARTIAL

udp: gso: Use single MSS length in UDP header for GSO_PARTIAL

In GSO_PARTIAL segmentation, set the UDP length field to the single
segment size (gso_size + UDP header) instead of the large MSS size.
This provides hardware with a template length value for final
segmentation, similar to how tunnel GSO_PARTIAL handles outer headers
in UDP tunnels.

This will remove the need to manually adjust the UDP header length in
the drivers, as can be seen in subsequent patches.

This was suggested by Alex in 2018:
https://lore.kernel.org/netdev/CAKgT0UcdnUWgr3KQ=RnLKigokkiUuYefmL-ePpDvJOBNpKScFA@mail.gmail.com/

Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260125121649.778086-2-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>