kernfs: update comment about kernfs_path() return value

Now it returns the length of the full path or error code.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 3abb1d90 ("kernfs: make kernfs_path*() behave in the style of strlcpy()")
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>