ipv6: protect flow label renew against GC

Take ip6_fl_lock before to read and update
a label.

v2: protect only the relevant code

Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
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>