net: Push capable(CAP_NET_ADMIN) into the rtnl methods

- In rtnetlink_rcv_msg convert the capable(CAP_NET_ADMIN) check
  to ns_capable(net->user-ns, CAP_NET_ADMIN).  Allowing unprivileged
  users to make netlink calls to modify their local network
  namespace.

- In the rtnetlink doit methods add capable(CAP_NET_ADMIN) so
  that calls that are not safe for unprivileged users are still
  protected.

Later patches will remove the extra capable calls from methods
that are safe for unprivilged users.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dcb/dcbnl.c

@@ -1662,6 +1662,9 @@ static int dcb_doit(struct sk_buff *skb, struct nlmsghdr *nlh, void *arg)
 	struct nlmsghdr *reply_nlh = NULL;
 	const struct reply_func *fn;
 
+	if ((nlh->nlmsg_type == RTM_SETDCB) && !capable(CAP_NET_ADMIN))
+		return -EPERM;
+
 	if (!net_eq(net, &init_net))
 		return -EINVAL;