geneve: ensure the skb is writable before fixing its headers

geneve: ensure the skb is writable before fixing its headers

Make sure the IPv4/6 and UDP headers are writable before fixing them up in
geneve_post_decap_hint. As skb_ensure_writable can reallocate the skb linear
area, reload the GRO hint header pointer and only set the IPv4/6 header ones
after the call.

Fixes: fd0dd796576e ("geneve: use GRO hint option in the RX path")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260529144713.780938-1-atenart%40kernel.org
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260709125000.141092-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>