ref_tracker: use %p instead of %px in debugfs dentry name

As Kees points out, this is a kernel address leak, and debugging is
not a sufficiently good reason to expose the real kernel address.

Fixes: 65b584f5 ("ref_tracker: automatically register a file in debugfs for a ref_tracker_dir")
Reported-by: Kees Cook <kees@kernel.org>
Closes: https://lore.kernel.org/netdev/202507301603.62E553F93@keescook/


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>