rtnetlink: Create link directly in target net namespace

Make rtnl_newlink_create() create device in target namespace directly.
Avoid extra netns change when link netns is provided.

Device drivers has been converted to be aware of link netns, that is not
assuming device netns is and link netns is the same when ops->newlink()
is called.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250219125039.18024-12-shaw.leon@gmail.com


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