Take ip6_fl_lock before to read and update
a label.

v2: protect only the relevant code
Reported-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NFlorent Fourcot <florent.fourcot@enst-bretagne.fr>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>