DO-NOT-MERGE: mptcp: improve code coverage for CI

mptcp: use kmalloc on kasan build

  Helps detection UaF, which apparently kasan misses with kmem_cache
  allocator.

  We also need to always set the SOCK_RCU_FREE flag, to preserved the
  current code leveraging SLAB_TYPESAFE_BY_RCU. This latter change will
  make unreachable some existing errors path, but I don't see other
  options.

tcp: warn if tcp_done() is called on a closed socket

  This is an extra check mainly for the CIs: to make sure we don't call
  tcp_done() on an already closed socket as it happened in the past. If
  we do such call, better to catch the error earlier.

Co-developed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Matthieu Baerts <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>

net/ipv4/tcp.c

@@ -4484,6 +4484,8 @@ void tcp_done(struct sock *sk)
 	 */
 	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk, 1);
 
+	WARN_ON_ONCE(sk->sk_state == TCP_CLOSE);
+
 	if (sk->sk_state == TCP_SYN_SENT || sk->sk_state == TCP_SYN_RECV)
 		TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);