ref_tracker: eliminate the ref_tracker_dir name field

Now that we have dentries and the ability to create meaningful symlinks
to them, don't keep a name string in each tracker. Switch the output
format to print "class@address", and drop the name field.

Also, add a kerneldoc header for ref_tracker_dir_init().

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-9-24fc37ead144@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>