geneve: add GRO hint output path

geneve: add GRO hint output path

If a geneve egress packet contains nested UDP encap headers, add a geneve
option including the information necessary on the RX side to perform GRO
aggregation of the whole packets: the nested network and transport headers,
and the nested protocol type.

Use geneve option class `netdev`, already registered in the Network
Virtualization Overlay (NVO3) IANA registry:

https://www.iana.org/assignments/nvo3/nvo3.xhtml#Linux-NetDev.

To pass the GRO hint information across the different xmit path functions,
store them in the skb control buffer, to avoid adding additional arguments.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/aa614567f7bdb776d693041375bede4990a19649.1769011015.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>