CVE-2026-43186: ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data()

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

ipv6: ioam: fix heap buffer overflow in ioam6filltracedata()

On the receive path, ioam6filltracedata() uses trace->nodelen to decide how much data to write for each node. It trusts this field as-is from the incoming packet, with no consistency check against trace->type (the 24-bit field that tells which data items are present). A crafted packet can set nodelen=0 while setting type bits 0-21, causing the function to write ~100 bytes past the allocated region (into skbsharedinfo), which corrupts adjacent heap memory and leads to a kernel panic.

Add a shared helper ioam6tracecomputenodelen() in ioam6.c to derive the expected nodelen from the type field, and use it:

- in ioam6iptunnel.c (send path, existing validation) to replace the open-coded computation; - in exthdrs.c (receive path, ipv6hopioam) to drop packets whose nodelen is inconsistent with the type field, before any data is written.

Per RFC 9197, bits 12-21 are each short (4-octet) fields, so they are included in IOAM6MASKSHORTFIELDS (changed from 0xff100000 to 0xff1ffc00).