btrfs snapshot and subvolume support

Submitted by Chris Murphy

Link to original bug (#710156)

Description

Ability to do the following:

1. display subvolume information btrfs subvolume show <subvol>
2. list subvolumes btrfs subvolume list <mountpoint>
3. create/delete subvolumes btrfs subvolume create/delete <mountpoint> btrfs subvolume snapshot <currentsubvol> <pathtonewsubvolname>
4. Ability to mount subvolumes mount -o subvol=<pathtosubvol> <dev> <mountpoint>

• Subvolumes includes snapshots since they are subvolumes.

Version: 3.10.x