search for: btrfs_root_label

From: Anand Jain <anand.jain@oracle.com> (This patch is for the review/test not yet for the integration). Here is an implementation of the feature to add label to the subvolume and snapshots. Which would help sysadmin to better manager the subvol and snapshots. This can be done in two ways, one - using attr which is user land only changes but drawback is able to change the label