CVE-2026-68274: drm/xe/guc: Fix buffer overflow in steered register list allocation

In the Linux kernel, the following vulnerability has been resolved: drm/xe/guc: Fix buffer overflow in steered register list allocation The size calculation for the steered register extarray uses only the geometry DSS mask (g_dss_mask) to determine the number of entries to allocate: total = bitmap_weight(gt->fuse_topo.g_dss_mask, ...) * steer_reg_num; However, the filling loop uses for_each_dss_steering(), which iterates over for_each_dss(), defined as the union of g_dss_mask and c_dss_mask (geometry + compute DSS). On platforms with compute-only DSS bits, the loop writes past the allocated buffer, corrupting adjacent slab objects. This manifests as list_del corruption and SLUB redzone overwrites during drm_managed_release on device unbind, since the overflow corrupts the drmres list_head of neighboring allocations. Fix by computing the allocation size using the union of both DSS masks, matching the iteration pattern of for_each_dss_steering(). -- v2: - use bitmap_weighted_or() (Zhanjun) (cherry picked from commit 0a78a44f4901aa6c9263e66be7fce02282f1109f)