mtd: blkdevs: do not forget to get MTD devices

Nowadays MTD devices have to be "get" before they can be
used. This has to be done with 'get_mtd_device()'. The
'blktrans_open()' function did not do this and instead
used 'try_module_get()'. Fix this.

Since 'get_mtd_device()' already gets the module, extra
'try_module_get()' is not needed.

This fixes oops when one tries to use mtdblock on top of
gluebi.

Reported-by: Holger Brunck <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>
Signed-off-by: David Woodhouse <[e-mail]>

drivers/mtd/mtd_blkdevs.c

···
 	struct mtd_blktrans_ops *tr = dev->tr;
 	int ret = -ENODEV;
 
-	if (!try_module_get(dev->mtd->owner))
+	if (!get_mtd_device(NULL, dev->mtd->index))
 		goto out;
 
 	if (!try_module_get(tr->owner))
···
 	ret = 0;
 	if (tr->open && (ret = tr->open(dev))) {
 		dev->mtd->usecount--;
-		module_put(dev->mtd->owner);
+		put_mtd_device(dev->mtd);
 	out_tr:
 		module_put(tr->owner);
 	}
···
 
 	if (!ret) {
 		dev->mtd->usecount--;
-		module_put(dev->mtd->owner);
+		put_mtd_device(dev->mtd);
 		module_put(tr->owner);
 	}