dm: add missing del_gendisk to alloc_dev error path

commit 03022c54 upstream.

Add missing del_gendisk() to error path when creation of workqueue fails.
Otherwice there is a resource leak and following warning is shown:

WARNING: at fs/sysfs/dir.c:487 sysfs_add_one+0xc5/0x160()
sysfs: cannot create duplicate filename '/devices/virtual/block/dm-0'
Signed-off-by: Zdenek Kabelac <zkabelac@redhat.com>
Reviewed-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/md/dm.c

@@ -1819,6 +1819,7 @@ static struct mapped_device *alloc_dev(int minor)
 bad_bdev:
 	destroy_workqueue(md->wq);
 bad_thread:
+	del_gendisk(md->disk);
 	put_disk(md->disk);
 bad_disk:
 	blk_cleanup_queue(md->queue);