truss: track Netlink socket protocols

Record the Netlink protocol associated with AF_NETLINK sockets when
they are created and pass it to libsysdecode during message decoding.

Use the protocol to distinguish between Generic Netlink and Route
Netlink sockets, ensuring that Generic Netlink decoding is only
performed for NETLINK_GENERIC sockets.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Reviewed by: kp
Sponsored-by : Google LLC (GSoC 2026)