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>

net/ipv6/ip6_flowlabel.c

@@ -464,7 +464,7 @@ static inline void fl_link(struct ipv6_pinfo *np, struct ipv6_fl_socklist *sfl,
 
 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;