net: stmmac: fix kernel panic due to NULL pointer dereference of buf->xdp

Ensure a valid XSK buffer before proceed to free the xdp buffer.

The following kernel panic is observed without this patch:

RIP: 0010:xp_free+0x5/0x40
Call Trace:
stmmac_napi_poll_rxtx+0x332/0xb30 [stmmac]
? stmmac_tx_timer+0x3c/0xb0 [stmmac]
net_rx_action+0x13d/0x3d0
__do_softirq+0xfc/0x2fb
? smpboot_register_percpu_thread+0xe0/0xe0
run_ksoftirqd+0x32/0x70
smpboot_thread_fn+0x1d8/0x2c0
kthread+0x169/0x1a0
? kthread_park+0x90/0x90
ret_from_fork+0x1f/0x30
---[ end trace 0000000000000002 ]---

Fixes: bba2556efa ("net: stmmac: Enable RX via AF_XDP zero-copy")
Cc: <stable@vger.kernel.org> # 5.13.x
Suggested-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -4914,6 +4914,10 @@ read_again:
 
 		prefetch(np);
 
+		/* Ensure a valid XSK buffer before proceed */
+		if (!buf->xdp)
+			break;
+
 		if (priv->extend_desc)
 			stmmac_rx_extended_status(priv, &priv->dev->stats,
 						  &priv->xstats,
@@ -4934,10 +4938,6 @@ read_again:
 			continue;
 		}
 
-		/* Ensure a valid XSK buffer before proceed */
-		if (!buf->xdp)
-			break;
-
 		/* XSK pool expects RX frame 1:1 mapped to XSK buffer */
 		if (likely(status & rx_not_ls)) {
 			xsk_buff_free(buf->xdp);