lightnvm: disable the subsystem

In commit 9ea9b9c4

 ("remove the lightnvm subsystem") the lightnvm
subsystem was removed as there is no hardware in the wild for it, and
the code is known to have problems.  This should also be disabled for
older LTS kernels as well to prevent anyone from accidentally using it.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <mb@lightnvm.io>
Cc: Javier González <javier@javigon.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>