tcp: handle shift/merge of cloned skbs too

This caused me to get repeatably:

  tcpdump: pcap_loop: recvfrom: Bad address

Happens occassionally when I tcpdump my for-looped test xfers:
  while [ : ]; do echo -n "$(date '+%s.%N') "; ./sendfile; sleep 20; done

Rest of the relevant commands:
  ethtool -K eth0 tso off
  tc qdisc add dev eth0 root netem drop 4%
  tcpdump -n -s0 -i eth0 -w sacklog.all

Running net-next under kvm, connection goes to the same host
(basically just out of kvm). The connection itself works ok
and data gets sent without corruption even with a large
number of tests while tcpdump fails usually within less than
5 tests.

Whether it only happens because of this change or not, I
don't know for sure but it's the only thing with which
I've seen that error. The non-cloned variant works w/o it
for much longer time. I'm yet to debug where the error
actually comes from.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -2018,13 +2018,10 @@ void skb_split(struct sk_buff *skb, struct sk_buff *skb1, const u32 len)
 		skb_split_no_header(skb, skb1, len, pos);
 }
 
-/* Shifting from/to a cloned skb is a no-go.
- *
- * TODO: handle cloned skbs by using pskb_expand_head()
- */
+/* Shifting from/to a cloned skb is a no-go. */
 static int skb_prepare_for_shift(struct sk_buff *skb)
 {
-	return skb_cloned(skb);
+	return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
 }
 
 /**