UBUNTU-CVE-2026-97953: Medium severity ubuntu/linux-azure vulnerability

In the Linux kernel, the following vulnerability has been resolved: net: stmmac: fix TX descriptor availability check for TSO traffic stmmactsoxmit() estimates the number of free TX descriptors required by a TSO skb as: (skb->len - protohdrlen) / TSOMAXBUFFSIZE + 1 which assumes the payload is split into TSOMAXBUFFSIZE chunks. This underestimates the descriptors actually consumed by stmmactsoallocator(), since each fragment is mapped individually and so it needs at least one descriptor regardless of its size. Moreover, one descriptor is used for the L2/L3/L4 headers and, when the MSS changes, one more is consumed for the MSS context descriptor. For a highly fragmented TSO skb the check can therefore pass even when the ring has too few free slots. stmmactsoallocator() then writes past the available descriptors, overwriting descriptors still owned by the DMA engine, corrupting the TX ring. Add stmmactsogetnumdesc() to compute the exact number of descriptors needed for the header, the linear payload and each fragment, plus the MSS context descriptor when required, and use it in the availability check.