net: ipv6: Use link netns in newlink() of rtnl_link_ops

When link_net is set, use it as link netns instead of dev_net(). This
prepares for rtnetlink core to create device in target netns directly,
in which case the two namespaces may be different.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250219125039.18024-9-shaw.leon@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>