mptcp: un-expose skb_clone_fraglist()

skb_clone_fraglist() had been exposed for MPTCP a while ago in
commit f5ad780 ("Fix: Create full new skb for the subflow").

But later, the function using it from MPTCP side has been removed in
commit a4b4ed0 ("mptcp: remove mptcp_pskb_copy").

We can then reduce the difference with upstream by reverting the
modifications around skb_clone_fraglist().

Fixes: a4b4ed0 ("mptcp: remove mptcp_pskb_copy")
Acked-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit a670e50)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit df7dd2c)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit a47d1cd)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

include/net/tcp.h

@@ -435,7 +435,6 @@ unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
 				       int large_allowed);
 u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb);
 
-void skb_clone_fraglist(struct sk_buff *skb);
 void copy_skb_header(struct sk_buff *new, const struct sk_buff *old);
 
 void inet_twsk_free(struct inet_timewait_sock *tw);

net/core/skbuff.c

@@ -545,7 +545,7 @@ static inline void skb_drop_fraglist(struct sk_buff *skb)
 	skb_drop_list(&skb_shinfo(skb)->frag_list);
 }
 
-void skb_clone_fraglist(struct sk_buff *skb)
+static void skb_clone_fraglist(struct sk_buff *skb)
 {
 	struct sk_buff *list;