ref_tracker: add a way to create a symlink to the ref_tracker_dir debugfs file

ref_tracker: add a way to create a symlink to the ref_tracker_dir debugfs file

Add the ability for a subsystem to add a user-friendly symlink that
points to a ref_tracker_dir's debugfs file. Add a separate
debugfs_symlinks xarray and use that to track symlinks. The reaper
workqueue job will remove symlinks before their corresponding dentries.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-7-24fc37ead144@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>