block: fix error handling in sg_io

Before commit 2cada584 ("block: cleanup error handling in sg_io"),
we had ret = 0 before entering the last big if block of sg_io.

Since 2cada584, ret = -EFAULT, which breaks hdparm:

/dev/sda:
 setting Advanced Power Management level to 0xc8 (200)
 HDIO_DRIVE_CMD failed: Bad address
 APM_level      = 128
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Fixes: 2cada584 ("block: cleanup error handling in sg_io")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/scsi_ioctl.c

@@ -330,6 +330,7 @@ static int sg_io(struct request_queue *q, struct gendisk *bd_disk,
 	if (blk_fill_sghdr_rq(q, rq, hdr, mode))
 		goto out_free_cdb;
 
+	ret = 0;
 	if (hdr->iovec_count) {
 		size_t iov_data_len;
 		struct iovec *iov = NULL;