null_blk: validate the number of devices

A negative number of devices is nonsensical, so change the type to
unsigned. If the number of devices is 0, it is impossible for userspace
to interact with the module, so refuse loading the driver for that case.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>