[NETNS][IPV6] flowlabels - make proc per namespace

Make /proc/net/ip6_flowlabel show only flow labels belonging to the
current network namespace.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>