mtd: mtd_blkdevs: Initialize rq.limits.discard_granularity

Since commit b35fd742("block: check queue's limits.discard_granularity
in __blkdev_issue_discard()") checks rq.limits.discard_granularity in
__blkdev_issue_discard(), we may get following warnings on formatted ftl:

  WARNING: CPU: 2 PID: 7313 at block/blk-lib.c:51
  __blkdev_issue_discard+0x2a7/0x390

Reproducer:
  1. ftl_format /dev/mtd0
  2. modprobe ftl
  3. mkfs.vfat /dev/ftla
  4. mount -odiscard /dev/ftla temp
  5. dd if=/dev/zero of=temp/tst bs=1M count=10 oflag=direct
  6. dd if=/dev/zero of=temp/tst bs=1M count=10 oflag=direct

Fix it by initializing rq.limits.discard_granularity if device supports
discard operation.
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210615093905.3473709-1-chengzhihao1@huawei.com

drivers/mtd/mtd_blkdevs.c

@@ -419,6 +419,7 @@ int add_mtd_blktrans_dev(struct mtd_blktrans_dev *new)
 	if (tr->discard) {
 		blk_queue_flag_set(QUEUE_FLAG_DISCARD, new->rq);
 		blk_queue_max_discard_sectors(new->rq, UINT_MAX);
+		new->rq->limits.discard_granularity = tr->blksize;
 	}
 
 	gd->queue = new->rq;