Merge branch 'netpoll-factor-out-functions-from-netpoll_send_udp-and-add-ipv6-selftest'

Breno Leitao says:

====================
netpoll: Factor out functions from netpoll_send_udp() and add ipv6 selftest

Refactors the netpoll UDP transmit path to improve code clarity,
maintainability, and protocol-layer encapsulation.

Function netpoll_send_udp() has more than 100 LoC, which is hard to
understand and review. After this patchset, it has only 32 LoC, which is
more manageable.

The series systematically moves the construction of protocol headers
(UDP, IPv4, IPv6, Ethernet) out of the core `netpoll_send_udp()`
function into dedicated static helpers:

  - `push_udp()` for UDP header setup
  - `push_ipv4()` and `push_ipv6()` for IP header setup
  - `push_eth()` for Ethernet header setup

This results in a clean, layered abstraction that mirrors the protocol
stack, reduces code duplication, and improves readability.

Also, to make sure this is not breaking anything, add IPv6 selftest to
netconsole tests, which will exercise this code. This test would also pick
problems similiar to the one fixed by f599020702698  ("net: netpoll:
Initialize UDP checksum field before checksumming"), which was
embarrassin we didn't have a selftest catch it.

Anyway, there are **no functional changes** intended in this patchset.

v1: https://lore.kernel.org/20250627-netpoll_untagle_ip-v1-0-61a21692f84a@debian.org
====================

Link: https://patch.msgid.link/20250702-netpoll_untagle_ip-v2-0-13cf3db24e2b@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>