nouveau/gsp: Avoid addressing beyond end of rpc->entries

Using the end of rpc->entries[] for addressing runs into both compile-time
and run-time detection of accessing beyond the end of the array. Use the
base pointer instead, since was allocated with the additional bytes for
storing the strings. Avoids the following warning in future GCC releases
with support for __counted_by:

In function 'fortify_memcpy_chk',
    inlined from 'r535_gsp_rpc_set_registry' at ../drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1123:3:
../include/linux/fortify-string.h:553:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  553 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

for this code:

	strings = (char *)&rpc->entries[NV_GSP_REG_NUM_ENTRIES];
	...
                memcpy(strings, r535_registry_entries[i].name, name_len);
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240330141159.work.063-kees@kernel.org

drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c

@@ -1112,7 +1112,7 @@ r535_gsp_rpc_set_registry(struct nvkm_gsp *gsp)
 	rpc->numEntries = NV_GSP_REG_NUM_ENTRIES;
 
 	str_offset = offsetof(typeof(*rpc), entries[NV_GSP_REG_NUM_ENTRIES]);
-	strings = (char *)&rpc->entries[NV_GSP_REG_NUM_ENTRIES];
+	strings = (char *)rpc + str_offset;
 	for (i = 0; i < NV_GSP_REG_NUM_ENTRIES; i++) {
 		int name_len = strlen(r535_registry_entries[i].name) + 1;