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 <stijn@linux-ipv6.be>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>