geneve: extract hint option at GRO stage

geneve: extract hint option at GRO stage

Add helpers for finding a GRO hint option in the geneve header, performing
basic sanitization of the option offsets vs the actual packet layout,
validate the option for GRO aggregation and check the nested header
checksum.

The validation helper closely mirrors similar check performed by the ipv4
and ipv6 gro callbacks, with the additional twist of accessing the
relevant network header via the GRO hint offset.

To validate the nested UDP checksum, leverage the csum completed of the
outer header, similarly to LCO, with the main difference that in this case
we have the outer checksum available.

Use the helpers to extract the hint info at the GRO stage.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/cd0e9dc42ba83f388b604097cffe268ffcb53351.1769011015.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>