[NET]: Avoid allocating skb in skb_pad

First of all it is unnecessary to allocate a new skb in skb_pad since
the existing one is not shared.  More importantly, our hard_start_xmit
interface does not allow a new skb to be allocated since that breaks
requeueing.

This patch uses pskb_expand_head to expand the existing skb and linearize
it if needed.  Actually, someone should sift through every instance of
skb_pad on a non-linear skb as they do not fit the reasons why this was
originally created.

Incidentally, this fixes a minor bug when the skb is cloned (tcpdump,
TCP, etc.).  As it is skb_pad will simply write over a cloned skb.  Because
of the position of the write it is unlikely to cause problems but still
it's best if we don't do it.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sk98lin/skge.c

@@ -1525,7 +1525,7 @@ struct sk_buff	*pMessage)	/* pointer to send-message              */
 	** This is to resolve faulty padding by the HW with 0xaa bytes.
 	*/
 	if (BytesSend < C_LEN_ETHERNET_MINSIZE) {
-		if ((pMessage = skb_padto(pMessage, C_LEN_ETHERNET_MINSIZE)) == NULL) {
+		if (skb_padto(pMessage, C_LEN_ETHERNET_MINSIZE)) {
 			spin_unlock_irqrestore(&pTxPort->TxDesRingLock, Flags);
 			return 0;
 		}