nouveau/gsp: replace zero-length array with flex-array member and use __counted_by

Fake flexible arrays (zero-length and one-element arrays) are deprecated,
and should be replaced by flexible-array members. So, replace
zero-length array with a flexible-array member in `struct
PACKED_REGISTRY_TABLE`.

Also annotate array `entries` with `__counted_by()` to prepare for the
coming implementation by GCC and Clang of the `__counted_by` attribute.
Flexible array members annotated with `__counted_by` can have their
accesses bounds-checked at run-time via `CONFIG_UBSAN_BOUNDS` (for array
indexing) and `CONFIG_FORTIFY_SOURCE` (for strcpy/memcpy-family functions).

This fixes multiple -Warray-bounds warnings:
drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1069:29: warning: array subscript 0 is outside array bounds of 'PACKED_REGISTRY_ENTRY[0]' [-Warray-bounds=]
drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1070:29: warning: array subscript 0 is outside array bounds of 'PACKED_REGISTRY_ENTRY[0]' [-Warray-bounds=]
drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1071:29: warning: array subscript 0 is outside array bounds of 'PACKED_REGISTRY_ENTRY[0]' [-Warray-bounds=]
drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1072:29: warning: array subscript 0 is outside array bounds of 'PACKED_REGISTRY_ENTRY[0]' [-Warray-bounds=]

While there, also make use of the struct_size() helper, and address
checkpatch.pl warning:
WARNING: please, no spaces at the start of a line

This results in no differences in binary output.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ZVZbX7C5suLMiBf+@work

drivers/gpu/drm/nouveau/include/nvrm/535.113.01/nvidia/generated/g_os_nvoc.h

@@ -38,7 +38,7 @@ typedef struct PACKED_REGISTRY_TABLE
 {
     NvU32                   size;
     NvU32                   numEntries;
-    PACKED_REGISTRY_ENTRY   entries[0];
+    PACKED_REGISTRY_ENTRY   entries[] __counted_by(numEntries);
 } PACKED_REGISTRY_TABLE;
 
 #endif

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

@@ -1046,7 +1046,7 @@ r535_gsp_rpc_set_registry(struct nvkm_gsp *gsp)
 	char *strings;
 	int str_offset;
 	int i;
-	size_t rpc_size = sizeof(*rpc) + sizeof(rpc->entries[0]) * NV_GSP_REG_NUM_ENTRIES;
+	size_t rpc_size = struct_size(rpc, entries, NV_GSP_REG_NUM_ENTRIES);
 
 	/* add strings + null terminator */
 	for (i = 0; i < NV_GSP_REG_NUM_ENTRIES; i++)