ipv6: increase maximum lifetime of flow labels

If the last RFC 6437 does not give any constraints
for lifetime of flow labels, the previous RFC 3697
spoke of a minimum of 120 seconds between
reattribution of a flow label.

The maximum linger is currently set to 60 seconds
and does not allow this configuration without
CAP_NET_ADMIN right.

This patch increase the maximum linger to 150
seconds, allowing more flexibility to standard
users.

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