CVE-2026-72098: dm-verity: fix buffer overflow in FEC calculation

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

dm-verity: fix buffer overflow in FEC calculation

There's a buffer overflow in dm-verity-fec:

if (neras && neras <= v->fec->roots) fio->erasures[(neras)++] = i;

This allows neras to reach roots + 1 (the post-increment pushes it past roots). This value is then passed as noeras to decoders8(). Inside the RS decoder (lib/reedsolomon/decoders.c:113-121), the erasure locator polynomial loop writes lambda[j] where j can reach nroots + 1 — one element past the end of lambda[] (which is sized nroots + 1, valid indices 0..nroots). The out-of-bounds write lands on syn[0], corrupting the syndrome buffer.