net: sit: require CAP_NET_ADMIN in the device netns for changelink

net: sit: require CAP_NET_ADMIN in the device netns for changelink

ipip6_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 t->net can rewrite a tunnel that
lives in t->net.

Gate ipip6_changelink() on rtnl_dev_link_net_capable() at its top,
before any attribute is parsed. sit was the one tunnel type not covered
by the recent series that added this check to the other changelink()
handlers.

Fixes: 5e6700b3bf98 ("sit: add support of x-netns")
Link: https://lore.kernel.org/netdev/20260612085941.3158249-1-maoyixie.tju@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260618070817.3378283-1-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>