[NET]: Ensure device name passed to SO_BINDTODEVICE is NULL terminated.

Found by Solar Designer.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -404,8 +404,9 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 			if (!valbool) {
 				sk->sk_bound_dev_if = 0;
 			} else {
-				if (optlen > IFNAMSIZ) 
-					optlen = IFNAMSIZ; 
+				if (optlen > IFNAMSIZ - 1)
+					optlen = IFNAMSIZ - 1;
+				memset(devname, 0, sizeof(devname));
 				if (copy_from_user(devname, optval, optlen)) {
 					ret = -EFAULT;
 					break;