debugfs: update comment of debugfs_rename()

Since commit ff9fb72 ("debugfs: return error values, not NULL")
changed return value of debugfs_rename() in error cases from %NULL to
%ERR_PTR(-ERROR), the comment of debugfs_rename should also be updated
so as not to mislead readers.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Link: https://lore.kernel.org/r/20230208035634.58095-1-zhengqi.arch@bytedance.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Qi Zheng

fs/debugfs/inode.c

@@ -802,8 +802,8 @@ EXPORT_SYMBOL_GPL(debugfs_lookup_and_remove);
  * exist for rename to succeed.
  *
  * This function will return a pointer to old_dentry (which is updated to
- * reflect renaming) if it succeeds. If an error occurs, %NULL will be
- * returned.
+ * reflect renaming) if it succeeds. If an error occurs, %ERR_PTR(-ERROR)
+ * will be returned.
  *
  * If debugfs is not enabled in the kernel, the value -%ENODEV will be
  * returned.