net: hsr: Use correct offset for HSR TLV values in supervisory HSR frames

Current HSR implementation uses following supervisory frame (even for
HSRv1 the HSR tag is not is not present):

00000000: 01 15 4e 00 01 2d XX YY ZZ 94 77 10 88 fb 00 01
00000010: 7e 1c 17 06 XX YY ZZ 94 77 10 1e 06 XX YY ZZ 94
00000020: 77 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000030: 00 00 00 00 00 00 00 00 00 00 00 00

The current code adds extra two bytes (i.e. sizeof(struct hsr_sup_tlv))
when offset for skb_pull() is calculated.
This is wrong, as both 'struct hsrv1_ethhdr_sp' and 'hsrv0_ethhdr_sp'
already have 'struct hsr_sup_tag' defined in them, so there is no need
for adding extra two bytes.

This code was working correctly as with no RedBox support, the check for
HSR_TLV_EOT (0x00) was off by two bytes, which were corresponding to
zeroed padded bytes for minimal packet size.

Fixes: eafaa88b ("net: hsr: Add support for redbox supervision frames")
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240228085644.3618044-1-lukma@denx.deSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/hsr/hsr_forward.c

@@ -83,7 +83,7 @@ static bool is_supervision_frame(struct hsr_priv *hsr, struct sk_buff *skb)
 		return false;
 
 	/* Get next tlv */
-	total_length += sizeof(struct hsr_sup_tlv) + hsr_sup_tag->tlv.HSR_TLV_length;
+	total_length += hsr_sup_tag->tlv.HSR_TLV_length;
 	if (!pskb_may_pull(skb, total_length))
 		return false;
 	skb_pull(skb, total_length);