mptcp: reset last_snd on subflow close

Send logic caches last active subflow in the msk, so it needs to be
cleared when the cached subflow is closed.

Fixes: d5f4919 ("mptcp: allow picking different xmit subflows")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/155
Reported-by: Christoph Paasch <cpaasch@apple.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -2126,6 +2126,8 @@ static struct sock *mptcp_subflow_get_retrans(const struct mptcp_sock *msk)
 static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 			      struct mptcp_subflow_context *subflow)
 {
+	struct mptcp_sock *msk = mptcp_sk(sk);
+
 	list_del(&subflow->node);
 
 	lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
@@ -2154,6 +2156,9 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 	release_sock(ssk);
 
 	sock_put(ssk);
+
+	if (ssk == msk->last_snd)
+		msk->last_snd = NULL;
 }
 
 void mptcp_close_ssk(struct sock *sk, struct sock *ssk,