net_sched: fix use of uninitialized ethertype variable in cls_flower

The addition of VLAN support caused a possible use of uninitialized
data if we encounter a zero TCA_FLOWER_KEY_ETH_TYPE key, as pointed
out by "gcc -Wmaybe-uninitialized":

net/sched/cls_flower.c: In function 'fl_change':
net/sched/cls_flower.c:366:22: error: 'ethertype' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This changes the code to only set the ethertype field if it
was nonzero, as before the patch.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9399ae9a ("net_sched: flower: Add vlan support")
Cc: Hadar Hen Zion <hadarh@mellanox.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_flower.c

@@ -353,7 +353,7 @@ static int fl_set_key(struct net *net, struct nlattr **tb,
 		       mask->eth.src, TCA_FLOWER_KEY_ETH_SRC_MASK,
 		       sizeof(key->eth.src));
 
-	if (tb[TCA_FLOWER_KEY_ETH_TYPE])
+	if (tb[TCA_FLOWER_KEY_ETH_TYPE]) {
 		ethertype = nla_get_be16(tb[TCA_FLOWER_KEY_ETH_TYPE]);
 
 		if (ethertype == htons(ETH_P_8021Q)) {
@@ -366,6 +366,7 @@ static int fl_set_key(struct net *net, struct nlattr **tb,
 			key->basic.n_proto = ethertype;
 			mask->basic.n_proto = cpu_to_be16(~0);
 		}
+	}
 
 	if (key->basic.n_proto == htons(ETH_P_IP) ||
 	    key->basic.n_proto == htons(ETH_P_IPV6)) {