Revert "ax25: zero length frame filtering in AX25"

This reverts commit f99bcff7.

Like netrom, Alan Cox says that zero lengths have real meaning
and are useful in this protocol.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -1435,11 +1435,6 @@ static int ax25_sendmsg(struct kiocb *iocb, struct socket *sock,
 	size_t size;
 	int lv, err, addr_len = msg->msg_namelen;
 
-	/* AX.25 empty data frame has no meaning : don't send */
-	if (len == 0) {
-		return (0);
-	}
-
 	if (msg->msg_flags & ~(MSG_DONTWAIT|MSG_EOR|MSG_CMSG_COMPAT))
 		return -EINVAL;
 
@@ -1639,13 +1634,6 @@ static int ax25_recvmsg(struct kiocb *iocb, struct socket *sock,
 	skb_reset_transport_header(skb);
 	copied = skb->len;
 
-	/* AX.25 empty data frame has no meaning : ignore it */
-	if (copied == 0) {
-		err = copied;
-		skb_free_datagram(sk, skb);
-		goto out;
-	}
-
 	if (copied > size) {
 		copied = size;
 		msg->msg_flags |= MSG_TRUNC;