ipv6: protect for_each_sk_fl_rcu in mem_check with rcu_read_lock_bh

Fixes a suspicious rcu derference warning.

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

net/ipv6/ip6_flowlabel.c

@@ -457,8 +457,10 @@ static int mem_check(struct sock *sk)
 	if (room > FL_MAX_SIZE - FL_MAX_PER_SOCK)
 		return 0;
 
+	rcu_read_lock_bh();
 	for_each_sk_fl_rcu(np, sfl)
 		count++;
+	rcu_read_unlock_bh();
 
 	if (room <= 0 ||
 	    ((count >= FL_MAX_PER_SOCK ||