net: correct comments of skb_shift

when skb_shift, we want to shift paged data from skb to tgt frag area.
Original comments revert the shift order
Signed-off-by: NFeng King <kinwin2008@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -2230,7 +2230,7 @@ static int skb_prepare_for_shift(struct sk_buff *skb)
  * @shiftlen: shift up to this many bytes
  *
  * Attempts to shift up to shiftlen worth of bytes, which may be less than
- * the length of the skb, from tgt to skb. Returns number bytes shifted.
+ * the length of the skb, from skb to tgt. Returns number bytes shifted.
  * It's up to caller to free skb if everything was shifted.
  *
  * If @tgt runs out of frags, the whole operation is aborted.