drm/nouveau/bl: increase buffer size to avoid truncate warning

The nouveau_get_backlight_name() function generates a unique name for the
backlight interface, appending an id from 1 to 99 for all backlight devices
after the first.

GCC 15 (and likely other compilers) produce the following
-Wformat-truncation warning:

nouveau_backlight.c: In function ‘nouveau_backlight_init’:
nouveau_backlight.c:56:69: error: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 3 [-Werror=format-truncation=]
56 | snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
| ^~
In function ‘nouveau_get_backlight_name’,
inlined from ‘nouveau_backlight_init’ at nouveau_backlight.c:351:7:
nouveau_backlight.c:56:56: note: directive argument in the range [1, 2147483647]
56 | snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
| ^~~~~~~~~~~~~~~~
nouveau_backlight.c:56:17: note: ‘snprintf’ output between 14 and 23 bytes into a destination of size 15
56 | snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The warning started appearing after commit ab244be47a8f ("drm/nouveau:
Fix a potential theorical leak in nouveau_get_backlight_name()") This fix
for the ida usage removed the explicit value check for ids larger than 99.
The compiler is unable to intuit that the ida_alloc_max() limits the
returned value range between 0 and 99.

Because the compiler can no longer infer that the number ranges from 0 to
99, it thinks that it could use as many as 11 digits (10 + the potential -
sign for negative numbers).

The warning has gone unfixed for some time, with at least one kernel test
robot report. The code breaks W=1 builds, which is especially frustrating
with the introduction of CONFIG_WERROR.

The string is stored temporarily on the stack and then copied into the
device name. Its not a big deal to use 11 more bytes of stack rounding out
to an even 24 bytes. Increase BL_NAME_SIZE to 24 to avoid the truncation
warning. This fixes the W=1 builds that include this driver.

Compile tested only.

Fixes: ab244be47a8f ("drm/nouveau: Fix a potential theorical leak in nouveau_get_backlight_name()")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312050324.0kv4PnfZ-lkp@intel.com/
Suggested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20250610-jk-nouveua-drm-bl-snprintf-fix-v2-1-7fdd4b84b48e@intel.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>