btrfs: remove the unnecessary result variables

Return the sysfs_emit() and iterate_object_props() directly instead of
using unnecessary variables.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: zhang songyi <zhang.songyi@zte.com.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>