net: gro: only merge packets with incrementing or fixed outer ids

Only merge encapsulated packets if their outer IDs are either
incrementing or fixed, just like for inner IDs and IDs of non-encapsulated
packets.

Add another ip_fixedid bit for a total of two bits: one for outer IDs (and
for unencapsulated packets) and one for inner IDs.

This commit preserves the current behavior of GSO where only the IDs of the
inner-most headers are restored correctly.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250923085908.4687-3-richardbgobert@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>