Merge branch 'adopting-nlmsg_payload-in-ipv4-ipv6'

Breno Leitao says:

====================
Adopting nlmsg_payload() in IPv4/IPv6

The commit 95d06e92a4019 ("netlink: Introduce nlmsg_payload helper")
introduced the nlmsg_payload() helper function.

This patchset aims to replace manual implementations with the
nlmsg_payload() helper in IPv4 and IPv6 files, one file per patch.
====================

Link: https://patch.msgid.link/20250415-nlmsg_v2-v1-0-a1c75d493fd7@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>