net: stmmac: Avoid VLA usage

In the quest to remove all stack VLAs from the kernel[1], this switches
the "status" stack buffer to use the existing small (8) upper bound on
how many queues can be checked for DMA, and adds a sanity-check just to
make sure it doesn't operate under pathological conditions.

[1] http://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2022,7 +2022,11 @@ static void stmmac_dma_interrupt(struct stmmac_priv *priv)
 				tx_channel_count : rx_channel_count;
 	u32 chan;
 	bool poll_scheduled = false;
-	int status[channels_to_check];
+	int status[max_t(u32, MTL_MAX_TX_QUEUES, MTL_MAX_RX_QUEUES)];
+
+	/* Make sure we never check beyond our status buffer. */
+	if (WARN_ON_ONCE(channels_to_check > ARRAY_SIZE(status)))
+		channels_to_check = ARRAY_SIZE(status);
 
 	/* Each DMA channel can be used for rx and tx simultaneously, yet
 	 * napi_struct is embedded in struct stmmac_rx_queue rather than in a