bonding: fix off-by-one error

Fix off-by-one error because IFNAMSIZ == 16 and when this
code gets executed we stick a NULL byte where we should not.

How to reproduce:
 with CONFIG_CC_STACKPROTECTOR=y (otherwise it may pass by silently)
 modprobe bonding; echo 1 > /sys/class/net/bond0/bonding/mode;
 echo "AAAAAAAAAAAAAAAA" > /sys/class/net/bond0/bonding/primary;
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_sysfs.c

@@ -1060,7 +1060,7 @@ static ssize_t bonding_store_primary(struct device *d,
 		goto out;
 	}
 
-	sscanf(buf, "%16s", ifname); /* IFNAMSIZ */
+	sscanf(buf, "%15s", ifname); /* IFNAMSIZ */
 
 	/* check to see if we are clearing primary */
 	if (!strlen(ifname) || buf[0] == '\n') {