mptcp: fix bad RCVPRUNED mib accounting

commit 0a567c2a10033bf04ed618368d179bce6977984b upstream.

Since its introduction, the mentioned MIB accounted for the wrong
event: wake-up being skipped as not-needed on some edge condition
instead of incoming skb being dropped after landing in the (subflow)
receive queue.

Move the increment in the correct location.

Fixes: ce599c5163 ("mptcp: properly account bulk freed memory")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ Conflicts in protocol.c, because the commit 6511882cdd ("mptcp:
  allocate fwd memory separately on the rx and tx path") is not in this
  version. The fix can still be applied before the 'goto drop'. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/mptcp/protocol.c

@@ -288,8 +288,10 @@ static bool __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 	if (!sk_rmem_schedule(sk, skb, skb->truesize)) {
 		int amount = sk_mem_pages(skb->truesize) << SK_MEM_QUANTUM_SHIFT;
 
-		if (ssk->sk_forward_alloc < amount)
+		if (ssk->sk_forward_alloc < amount) {
+			MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_RCVPRUNED);
 			goto drop;
+		}
 
 		ssk->sk_forward_alloc -= amount;
 		sk->sk_forward_alloc += amount;
@@ -774,10 +776,8 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 		sk_rbuf = ssk_rbuf;
 
 	/* over limit? can't append more skbs to msk, Also, no need to wake-up*/
-	if (__mptcp_rmem(sk) > sk_rbuf) {
-		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_RCVPRUNED);
+	if (__mptcp_rmem(sk) > sk_rbuf)
 		return;
-	}
 
 	/* Wake-up the reader only for in-sequence data */
 	mptcp_data_lock(sk);