lightnvm: fix up nonsensical configure overrun checking

Instead of checking a constant 0 actually check the space available. Even
better remember to allow for the header and also check the right amount of
space is needed.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>