Merge branch 'net-deduplicate-cookie-logic'

Willem de Bruijn says:

====================
net: deduplicate cookie logic

Reuse standard sk, ip and ipv6 cookie init handlers where possible.

Avoid repeated open coding of the same logic.
Harmonize feature sets across protocols.
Make IPv4 and IPv6 logic more alike.
Simplify adding future new fields with a single init point.
====================

Link: https://patch.msgid.link/20250214222720.3205500-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>