mptcp: make the symbol 'mptcp_sk_clone_lock' static

Fix the following sparse warning:
net/mptcp/protocol.c:646:13: warning: symbol 'mptcp_sk_clone_lock' was not declared. Should it be static?

Fixes: b0519de8b3 ("mptcp: fix use-after-free for ipv6")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -643,7 +643,7 @@ static struct ipv6_pinfo *mptcp_inet6_sk(const struct sock *sk)
 }
 #endif
 
-struct sock *mptcp_sk_clone_lock(const struct sock *sk)
+static struct sock *mptcp_sk_clone_lock(const struct sock *sk)
 {
 	struct sock *nsk = sk_clone_lock(sk, GFP_ATOMIC);