net: dsa: fix a crash if ->get_sset_count() fails

If ds->ops->get_sset_count() fails then it "count" is a negative error
code such as -EOPNOTSUPP.  Because "i" is an unsigned int, the negative
error code is type promoted to a very high value and the loop will
corrupt memory until the system crashes.

Fix this by checking for error codes and changing the type of "i" to
just int.

Fixes: badf3ada ("net: dsa: Provide CPU port statistics to master netdev")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/master.c

@@ -147,8 +147,7 @@ static void dsa_master_get_strings(struct net_device *dev, uint32_t stringset,
 	struct dsa_switch *ds = cpu_dp->ds;
 	int port = cpu_dp->index;
 	int len = ETH_GSTRING_LEN;
-	int mcount = 0, count;
-	unsigned int i;
+	int mcount = 0, count, i;
 	uint8_t pfx[4];
 	uint8_t *ndata;
 
@@ -178,6 +177,8 @@ static void dsa_master_get_strings(struct net_device *dev, uint32_t stringset,
 		 */
 		ds->ops->get_strings(ds, port, stringset, ndata);
 		count = ds->ops->get_sset_count(ds, port, stringset);
+		if (count < 0)
+			return;
 		for (i = 0; i < count; i++) {
 			memmove(ndata + (i * len + sizeof(pfx)),
 				ndata + i * len, len - sizeof(pfx));