Trying to build parisc:allmodconfig with gcc 12.x or later results
in the following build error.

drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_mthd':
drivers/gpu/drm/nouveau/nvif/object.c:161:9: error:
	'memcpy' accessing 4294967264 or more bytes at offsets 0 and 32 overlaps 6442450881 bytes at offset -2147483617 [-Werror=restrict]
  161 |         memcpy(data, args->mthd.data, size);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_ctor':
drivers/gpu/drm/nouveau/nvif/object.c:298:17: error:
	'memcpy' accessing 4294967240 or more bytes at offsets 0 and 56 overlaps 6442450833 bytes at offset -2147483593 [-Werror=restrict]
  298 |                 memcpy(data, args->new.data, size);

gcc assumes that 'sizeof(*args) + size' can overflow, which would result
in the problem.

The problem is not new, only it is now no longer a warning but an error
since W=1 has been enabled for the drm subsystem and since Werror is
enabled for test builds.

Rearrange arithmetic and use check_add_overflow() for validating the
allocation size to avoid the overflow. While at it, split assignments
out of if conditions.

Fixes: a61ddb43 ("drm: enable (most) W=1 warnings by default across the subsystem")
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Joe Perches <joe@perches.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240524134817.1369993-1-linux@roeck-us.net