kernfs: remove kernfs_path_len()

It doesn't have any in-kernel user and the same result can be obtained
from kernfs_path(@kn, NULL, 0).  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>