net: add symlinks to ref_tracker_dir for netns

After assigning the inode number to the namespace, use it to create a
unique name for each netns refcount tracker with the ns.inum and
net_cookie values in it, and register a symlink to the debugfs file for
it.

init_net is registered before the ref_tracker dir is created, so add a
late_initcall() to register its files and symlinks.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-8-24fc37ead144@kernel.org


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