mptcp: fix bad RCVPRUNED mib accounting

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: ce599c51 ("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>

net/mptcp/protocol.c

@@ -350,8 +350,10 @@ static bool __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 	skb_orphan(skb);
 
 	/* try to fetch required memory from subflow */
-	if (!mptcp_rmem_schedule(sk, ssk, skb->truesize))
+	if (!mptcp_rmem_schedule(sk, ssk, skb->truesize)) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_RCVPRUNED);
 		goto drop;
+	}
 
 	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
 
@@ -844,10 +846,8 @@ 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);