tcp: don't require root to read tcp_metrics

commit d23ff701 (tcp: add generic netlink support for tcp_metrics) introduced
netlink support for the new tcp_metrics, however it restricted getting of
tcp_metrics to root user only. This is a change from how these values could
have been fetched when in the old route cache. Unless there's a legitimate
reason to restrict the reading of these values it would be better if normal
users could fetch them.

Cc: Julian Anastasov <ja@ssi.bg>
Cc: linux-kernel@vger.kernel.org

Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>