Fix the following sparse warnings:

drivers/block/null_blk/main.c:1243:35: warning: incorrect type in return expression (different base types)
drivers/block/null_blk/main.c:1243:35:    expected int
drivers/block/null_blk/main.c:1243:35:    got restricted blk_status_t
drivers/block/null_blk/main.c:1291:30: warning: incorrect type in return expression (different base types)
drivers/block/null_blk/main.c:1291:30:    expected restricted blk_status_t
drivers/block/null_blk/main.c:1291:30:    got int

Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510201816.24921-1-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>