net: garp: fix unsigned integer underflow in garp_pdu_parse_attr

In the Linux kernel, the following vulnerability has been resolved:

net: garp: fix unsigned integer underflow in garp_pdu_parse_attr

The receive-side GARP attribute parser computes dlen with reversed

operands:

dlen = sizeof(*ga) - ga->len;

ga->len is the on-wire attribute length and includes the GARP attribute

header. For normal attributes with data, ga->len is larger than

sizeof(*ga), so the subtraction underflows in unsigned arithmetic.

The resulting value is later passed to garp_attr_lookup(), whose length

argument is u8. After truncation, the parsed data length usually no

longer matches the length stored for locally registered attributes, so

received Join/Leave events are ignored. This breaks the GARP receive path

for common attributes, such as GVRP VLAN registration attributes.

Compute the data length as the attribute length minus the header length.