mptcp: fix locking in mptcp_disconnect()

tcp_disconnect() expects the caller acquires the sock lock,
but mptcp_disconnect() is not doing that. Add the missing
required lock.
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: 76e2a55d ("mptcp: better msk-level shutdown.")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/f818e82b58a556feeb71dcccc8bf1c87aafc6175.1610638176.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -2646,8 +2646,13 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	struct mptcp_sock *msk = mptcp_sk(sk);
 
 	__mptcp_flush_join_list(msk);
-	mptcp_for_each_subflow(msk, subflow)
-		tcp_disconnect(mptcp_subflow_tcp_sock(subflow), flags);
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		lock_sock(ssk);
+		tcp_disconnect(ssk, flags);
+		release_sock(ssk);
+	}
 	return 0;
 }