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

this warning:

  net/ipv6/ip6_flowlabel.c: In function ‘ipv6_flowlabel_opt’:
  net/ipv6/ip6_flowlabel.c:467: warning: ‘err’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between fl_create() and 'err'.

Annotate it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index 7927a84..5656e8a 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c
@@ -464,7 +464,7 @@
 
 int ipv6_flowlabel_opt(struct sock *sk, char __user *optval, int optlen)
 {
-	int err;
+	int uninitialized_var(err);
 	struct net *net = sock_net(sk);
 	struct ipv6_pinfo *np = inet6_sk(sk);
 	struct in6_flowlabel_req freq;