mtd_blkdevs: don't scan partitions for plain mtdblock

mtdblock / mtdblock_ro set part_bits to 0 and thus nevever scanned
partitions.  Restore that behavior by setting the GENHD_FL_NO_PART flag.

Fixes: 1ebe2e5 ("block: remove GENHD_FL_EXT_DEVT")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211206070409.2836165-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

drivers/mtd/mtd_blkdevs.c

@@ -346,7 +346,7 @@ int add_mtd_blktrans_dev(struct mtd_blktrans_dev *new)
 	gd->minors = 1 << tr->part_bits;
 	gd->fops = &mtd_block_ops;
 
-	if (tr->part_bits)
+	if (tr->part_bits) {
 		if (new->devnum < 26)
 			snprintf(gd->disk_name, sizeof(gd->disk_name),
 				 "%s%c", tr->name, 'a' + new->devnum);
@@ -355,9 +355,11 @@ int add_mtd_blktrans_dev(struct mtd_blktrans_dev *new)
 				 "%s%c%c", tr->name,
 				 'a' - 1 + new->devnum / 26,
 				 'a' + new->devnum % 26);
-	else
+	} else {
 		snprintf(gd->disk_name, sizeof(gd->disk_name),
 			 "%s%d", tr->name, new->devnum);
+		gd->flags |= GENHD_FL_NO_PART;
+	}
 
 	set_capacity(gd, ((u64)new->size * tr->blksize) >> 9);