net: correct comment on where to place transmit time stamp hook.

The comment for the skb_tx_timestamp() function suggests calling it just
after a buffer is released to the hardware for transmission. However,
for drivers that free the buffer in an ISR, this produces a race between
the time stamp code and the ISR. This commit changes the comment to advise
placing the call just before handing the buffer over to the hardware.
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2023,8 +2023,7 @@ static inline void sw_tx_timestamp(struct sk_buff *skb)
  * skb_tx_timestamp() - Driver hook for transmit timestamping
  *
  * Ethernet MAC Drivers should call this function in their hard_xmit()
- * function as soon as possible after giving the sk_buff to the MAC
- * hardware, but before freeing the sk_buff.
+ * function immediately before giving the sk_buff to the MAC hardware.
  *
  * @skb: A socket buffer.
  */