debugfs_create_dir() returns error pointers.  It never returns NULL.  So
use IS_ERR() to check it.

Link: https://lkml.kernel.org/r/20240821073441.9701-1-11162571@vivo.comSigned-off-by: Yang Ruibin <11162571@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>