ipv6: add ip6_flowlabel helper

And use it if possible.

Signed-off-by: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ipv6.h
Simple merge

diff --cc net/ipv6/ip6_gre.c
Simple merge

diff --cc net/ipv6/ip6_tunnel.c
Simple merge