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 --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index f5e8a98..8d7c986 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1130,7 +1130,7 @@
 	if (t->parms.flags & IP6_TNL_F_USE_ORIG_TCLASS)
 		fl6.flowlabel |= (*(__be32 *) ipv6h & IPV6_TCLASS_MASK);
 	if (t->parms.flags & IP6_TNL_F_USE_ORIG_FLOWLABEL)
-		fl6.flowlabel |= (*(__be32 *) ipv6h & IPV6_FLOWLABEL_MASK);
+		fl6.flowlabel |= ip6_flowlabel(ipv6h);
 	if (t->parms.flags & IP6_TNL_F_USE_ORIG_FWMARK)
 		fl6.flowi6_mark = skb->mark;