bridge: mdb: avoid uninitialized variable warning

A recent change to the mdb code confused the compiler to the point
where it did not realize that the port-group returned from
br_mdb_add_group() is always valid when the function returns a nonzero
return value, so we get a spurious warning:

net/bridge/br_mdb.c: In function 'br_mdb_add':
net/bridge/br_mdb.c:542:4: error: 'pg' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    __br_mdb_notify(dev, entry, RTM_NEWMDB, pg);

Slightly rearranging the code in br_mdb_add_group() makes the problem
go away, as gcc is clever enough to see that both functions check
for 'ret != 0'.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Fixes: 9e8430f8 ("bridge: mdb: Passing the port-group pointer to br_mdb module")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_mdb.c

@@ -425,8 +425,8 @@ static int br_mdb_add_group(struct net_bridge *br, struct net_bridge_port *port,
 	mp = br_mdb_ip_get(mdb, group);
 	if (!mp) {
 		mp = br_multicast_new_group(br, port, group);
-		err = PTR_ERR(mp);
-		if (IS_ERR(mp))
+		err = PTR_ERR_OR_ZERO(mp);
+		if (err)
 			return err;
 	}