drm/xe: Fix uninitialized return value from xe_validation_guard()

the DEFINE_CLASS() macro creates an inline function and
the init args are passed down to it; since _ret is passed as an int,
whatever value is set inside the function is not visible to the caller.
Pass _ret as a pointer so its value propagates to the caller.

Fixes: c460bc2 ("drm/xe: Introduce an xe_validation wrapper around drm_exec")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6220
Cc: Maarten Lankhorst <maarten.lankhorst@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: intel-xe@lists.freedesktop.org
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251027131228.12098-1-thomas.hellstrom@linux.intel.com
(cherry picked from commit fcb8c30)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Author: Thomas Hellström

drivers/gpu/drm/xe/xe_validation.h

@@ -166,10 +166,10 @@ xe_validation_device_init(struct xe_validation_device *val)
  */
 DEFINE_CLASS(xe_validation, struct xe_validation_ctx *,
 	     if (_T) xe_validation_ctx_fini(_T);,
-	     ({_ret = xe_validation_ctx_init(_ctx, _val, _exec, _flags);
-	       _ret ? NULL : _ctx; }),
+	     ({*_ret = xe_validation_ctx_init(_ctx, _val, _exec, _flags);
+	       *_ret ? NULL : _ctx; }),
 	     struct xe_validation_ctx *_ctx, struct xe_validation_device *_val,
-	     struct drm_exec *_exec, const struct xe_val_flags _flags, int _ret);
+	     struct drm_exec *_exec, const struct xe_val_flags _flags, int *_ret);
 static inline void *class_xe_validation_lock_ptr(class_xe_validation_t *_T)
 {return *_T; }
 #define class_xe_validation_is_conditional true
@@ -186,7 +186,7 @@ static inline void *class_xe_validation_lock_ptr(class_xe_validation_t *_T)
  * exhaustive eviction.
  */
 #define xe_validation_guard(_ctx, _val, _exec, _flags, _ret)		\
-	scoped_guard(xe_validation, _ctx, _val, _exec, _flags, _ret) \
+	scoped_guard(xe_validation, _ctx, _val, _exec, _flags, &_ret) \
 	drm_exec_until_all_locked(_exec)
 
 #endif