kernel: fix kernel panic in mtk_ppe

The patch that adds support for hw flow-offloading counters on newer
MediaTek SoCs tries to prints acct->packets and acct->bytes in debugfs,
without checking that acct isn't null. This causes a kernel panic when
trying to read /sys/kernel/debug/ppe0/entries on older MediaTek SoCs.
Fix this by adding a check for acct.

Fixes: openwrt#11756
Fixes: 9721a42a27 ("kernel: support hw flow-offloading counters on newer MediaTek SoCs")
Reported-by: Stijn Tintel <[email protected]>
Signed-off-by: Daniel Golle <[email protected]>

target/linux/generic/pending-5.15/731-net-ethernet-mediatek-ppe-add-support-for-flow-accou.patch

@@ -357,7 +357,7 @@ v2: fix wrong variable name in return value check spotted by Denis Kirjanov
  			   h_source, h_dest, ntohs(l2->etype),
 -			   l2->vlan1, l2->vlan2, entry->ib1, ib2);
 +			   l2->vlan1, l2->vlan2, entry->ib1, ib2,
-+			   acct->packets, acct->bytes);
++			   acct ? acct->packets : 0, acct ? acct->bytes : 0);
  	}
  
  	return 0;