ref_tracker: add a static classname string to each ref_tracker_dir

ref_tracker: add a static classname string to each ref_tracker_dir

A later patch in the series will be adding debugfs files for each
ref_tracker that get created in ref_tracker_dir_init(). The format will
be "class@%px". The current "name" string can vary between
ref_tracker_dir objects of the same type, so it's not suitable for this
purpose.

Add a new "class" string to the ref_tracker dir that describes the
the type of object (sans any individual info for that object).

Also, in the i915 driver, gate the creation of debugfs files on whether
the dentry pointer is still set to NULL. CI has shown that the
ref_tracker_dir can be initialized more than once.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-4-24fc37ead144@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>