Merge branch 'net-require-cap_net_admin-in-the-device-netns-for-tunnel-changelink'

Maoyi Xie says:

====================
net: require CAP_NET_ADMIN in the device netns for tunnel changelink

A tunnel changelink() operates on at most two netns, dev_net(dev) and
the tunnel link netns t->net. They differ once the device is created in
or moved to a netns other than the one the request runs in. The rtnl
changelink path checks CAP_NET_ADMIN only against dev_net(dev), so a
caller privileged there but not in the link netns can rewrite a tunnel
that lives in the link netns. Commit 8b484efd5cb4 ("ip6: vti: Use
ip6_tnl.net in vti6_siocdevprivate().") added the same check on the
ioctl path. This series adds it on the RTM_NEWLINK path.

Each changelink is gated at the top of the op, before any attribute is
parsed, because the per-type parsers can update live tunnel fields first
(for example ipgre_netlink_parms() sets t->collect_md). The check is
skipped when the link netns is dev_net(dev), where the rtnl path already
checked the cap.

The check goes through a new helper, rtnl_dev_link_net_capable(), added
in patch 1 next to rtnl_get_net_ns_capable() in net/core/rtnetlink.c.

Tested on net/main. For every tunnel type in the series a migrated
fake-root changelink is rejected with EPERM. For vti6 SIOCGETTUNNEL
confirms the link netns hash is left unchanged. Legit non-migrated
changelinks still succeed.

v5: https://lore.kernel.org/20260611062814.2528793-1-maoyixie.tju@gmail.com
v4: https://lore.kernel.org/20260609163110.1717419-1-maoyixie.tju@gmail.com
v3: https://lore.kernel.org/20260604125055.3254652-1-maoyixie.tju@gmail.com
v2: https://lore.kernel.org/20260601034148.1272080-1-maoyixie.tju@gmail.com
v1: https://lore.kernel.org/20260527070824.2677331-1-maoyixie.tju@gmail.com
====================

Link: https://patch.msgid.link/20260612085941.3158249-1-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>