Merge branch 'net-introduce-nlmsg_payload-helper'

Breno Leitao says:

====================
net: Introduce nlmsg_payload helper

In the current codebase, there are multiple instances where the
structure size is checked before assigning it to a Netlink message. This
check is crucial for ensuring that the structure is correctly mapped
onto the Netlink message, providing a layer of security.

To streamline this process, Jakub Kicinski suggested creating a helper
function, `nlmsg_payload`, which verifies if the structure fits within
the message. If it does, the function returns the data; otherwise, it
returns NULL. This approach simplifies the code and reduces redundancy.

This patchset introduces the `nlmsg_payload` helper and updates several
parts of the code to use it. Further updates will follow in subsequent
patchsets.

v1: https://lore.kernel.org/20250411-nlmsg-v1-0-ddd4e065cb15@debian.org
====================

Link: https://patch.msgid.link/20250414-nlmsg-v2-0-3d90cb42c6af@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>