drm/xe: Fix access_ok check in user_fence_create

Check size of the data not size of the pointer.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407300421.IBkAja96-lkp@intel.com/
Fixes: 0fde907d ("drm/xe: Validate user fence during creation")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Apoorva Singh <apoorva.singh@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806110722.28661-1-nirmoy.das@intel.comSigned-off-by: Nirmoy Das <nirmoy.das@intel.com>

drivers/gpu/drm/xe/xe_sync.c

@@ -55,7 +55,7 @@ static struct xe_user_fence *user_fence_create(struct xe_device *xe, u64 addr,
 	struct xe_user_fence *ufence;
 	u64 __user *ptr = u64_to_user_ptr(addr);
 
-	if (!access_ok(ptr, sizeof(ptr)))
+	if (!access_ok(ptr, sizeof(*ptr)))
 		return ERR_PTR(-EFAULT);
 
 	ufence = kmalloc(sizeof(*ufence), GFP_KERNEL);