null_blk: match the type of parameter nr_devices

Since the variable nr_devices is an unsigned int, the module_param()
should also use this type. Change the type so they can match.

Fixes: f7c4ce89

 ("null_blk: validate the number of devices")
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>