drm: Fix potential overflow issue in event_string array

When calling scnprintf() to append recovery method to event_string,
the second argument should be `sizeof(event_string) - len`, otherwise
there is a potential overflow problem.

Fixes: b7cf9f4 ("drm: Introduce device wedged event")
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250409014633.31303-1-jiangfeng@kylinos.cn
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: flynnjiang

drivers/gpu/drm/drm_drv.c

@@ -549,7 +549,7 @@ int drm_dev_wedged_event(struct drm_device *dev, unsigned long method)
 		if (drm_WARN_ONCE(dev, !recovery, "invalid recovery method %u\n", opt))
 			break;
 
-		len += scnprintf(event_string + len, sizeof(event_string), "%s,", recovery);
+		len += scnprintf(event_string + len, sizeof(event_string) - len, "%s,", recovery);
 	}
 
 	if (recovery)