net: fix fraglist segmentation reference count leak

Xin Long says:
 On udp rx path udp_rcv_segment() may do segment where the frag skbs
 will get the header copied from the head skb in skb_segment_list()
 by calling __copy_skb_header(), which could overwrite the frag skbs'
 extensions by __skb_ext_copy() and cause a leak.

 This issue was found after loading esp_offload where a sec path ext
 is set in the skb.

Fix this by discarding head state of the fraglist skb before replacing
its contents.

Fixes: 3a1296a38d ("net: Support GRO/GSO fraglist chaining.")
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Reported-by: Xiumei Mu <xmu@redhat.com>
Tested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3668,6 +3668,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 
 		skb_push(nskb, -skb_network_offset(nskb) + offset);
 
+		skb_release_head_state(nskb);
 		 __copy_skb_header(nskb, skb);
 
 		skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));