rtnetlink: add missing netlink_ns_capable() check for peer netns

In the Linux kernel, the following vulnerability has been resolved:

rtnetlink: add missing netlink_ns_capable() check for peer netns

rtnl_newlink() lacks a CAP_NET_ADMIN capability check on the peer

network namespace when creating paired devices (veth, vxcan,

netkit). This allows an unprivileged user with a user namespace

to create interfaces in arbitrary network namespaces, including

init_net.

Add a netlink_ns_capable() check for CAP_NET_ADMIN in the peer

namespace before allowing device creation to proceed.