scsi: simplify scsi_prep_state_check

Return a blk_status_t directly, and make the code a little more compact
by handling the fast path in the caller.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>