rtnetlink: Try the outer netns attribute in rtnl_get_peer_net().

Xiao Liang reported that the cited commit changed netns handling
in newlink() of netkit, veth, and vxcan.

Before the patch, if we don't find a netns attribute in the peer
device attributes, we tried to find another netns attribute in
the outer netlink attributes by passing it to rtnl_link_get_net().

Let's restore the original behaviour.

Fixes: 48327566 ("rtnetlink: fix double call of rtnl_link_get_net_ifla()")
Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCORBVVU8P6AHcEkENMj+gD2d3ce9t=A_o48E0yOQp8_wUQ@mail.gmail.com/#t


Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Tested-by: Xiao Liang <shaw.leon@gmail.com>
Link: https://patch.msgid.link/20241216110432.51488-1-kuniyu@amazon.com


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