lightnvm: bad type conversion for nvme control bits

The NVMe I/O command control bits are 16 bytes, but is interpreted as
32 bytes in the lightnvm user I/O data path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>