ref_tracker: add a top level debugfs directory for ref_tracker

ref_tracker: add a top level debugfs directory for ref_tracker

Add a new "ref_tracker" directory in debugfs. Each individual refcount
tracker can register files under there to display info about
currently-held references.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-2-24fc37ead144@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>