geneve: fix length used in GRO hint UDP checksum adjustment

geneve: fix length used in GRO hint UDP checksum adjustment

In geneve_post_decap_hint the length used for adjusting the UDP checksum
should be 'skb->len - gro_hint->nested_tp_offset' (UDP length) instead
of 'skb->len - gro_hint->nested_nh_offset' (IP length).

Fixes: fd0dd796576e ("geneve: use GRO hint option in the RX path")
Cc: Paolo Abeni <pabeni@redhat.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260521131436.748832-1-jhs%40mojatatu.com
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260529144713.780938-1-atenart@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>