Btrfs: sysfs: fix, kobject pointer clean up needed after kobject release

The sysfs clean up self test like in the below code fails, since
fs_info->device_dir_kobject still points to its stale kobject.
Reseting this pointer will help to fix this.

open_ctree()
{

ret = btrfs_sysfs_add_one(fs_info);
::
+       btrfs_sysfs_remove_one(fs_info);
+       ret = btrfs_sysfs_add_one(fs_info);
+       if (ret) {
+               pr_err("BTRFS: failed to init sysfs interface: %d\n", ret);
+               goto fail_block_groups;
+       }

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/sysfs.c

@@ -525,6 +525,7 @@ void btrfs_sysfs_remove_one(struct btrfs_fs_info *fs_info)
 	btrfs_kobj_rm_device(fs_info, NULL);
 	kobject_del(fs_info->device_dir_kobj);
 	kobject_put(fs_info->device_dir_kobj);
+	fs_info->device_dir_kobj = NULL;
 	addrm_unknown_feature_attrs(fs_info, false);
 	sysfs_remove_group(&fs_info->super_kobj, &btrfs_feature_attr_group);
 	__btrfs_sysfs_remove_one(fs_info);