net: Add SPDX ids to some source files

net: Add SPDX ids to some source files

Add SPDX-License-Identifier lines to several source
files under the network sub-directory.  Work on files
in the core, dns_resolver, ipv4, ipv6 and
netfilter sub-dirs.  Remove boilerplate
and license reference text to avoid ambiguity.

Rusty Russell has expressed that his contributions
were intended to be GPL-2.0-or-later.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Link: https://patch.msgid.link/20260305004724.87469-1-tim.bird@sony.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>