ipv6: fix a RCU warning in net/ipv6/ip6_flowlabel.c

This patch fixes the following RCU warning:

[   51.680236] ===============================
[   51.681914] [ INFO: suspicious RCU usage. ]
[   51.683610] 3.8.0-rc6-next-20130206-sasha-00028-g83214f7-dirty #276 Tainted: G        W
[   51.686703] -------------------------------
[   51.688281] net/ipv6/ip6_flowlabel.c:671 suspicious rcu_dereference_check() usage!

we should use rcu_dereference_bh() when we hold rcu_read_lock_bh().

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>