kernfs: add dummy implementation of kernfs_path_from_node()

The dummy version of kernfs_path_from_node() was missing.  This
currently doesn't break anything.  Let's add it for consistency and to
ease adding wrappers around it.

v2: Removed stray ';' which was causing build failures.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>