mtd: ftl: use container_of() rather than cast

The container_of() is much more readable and also safer.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220816135910.268016-1-cuigaosheng1@huawei.com

drivers/mtd/ftl.c

@@ -941,7 +941,7 @@ static int ftl_write(partition_t *part, caddr_t buffer,
 
 static int ftl_getgeo(struct mtd_blktrans_dev *dev, struct hd_geometry *geo)
 {
-	partition_t *part = (void *)dev;
+	partition_t *part = container_of(dev, struct partition_t, mbd);
 	u_long sect;
 
 	/* Sort of arbitrary: round size down to 4KiB boundary */
@@ -969,7 +969,7 @@ static int ftl_writesect(struct mtd_blktrans_dev *dev,
 static int ftl_discardsect(struct mtd_blktrans_dev *dev,
 			   unsigned long sector, unsigned nr_sects)
 {
-	partition_t *part = (void *)dev;
+	partition_t *part = container_of(dev, struct partition_t, mbd);
 	uint32_t bsize = 1 << part->header.EraseUnitSize;
 
 	pr_debug("FTL erase sector %ld for %d sectors\n",