disable broken btrfs code.

`btrfs subvolume list /` does not output anything on my system.

`btrfs subvolume list -a /` results in:

	ID 256 gen 641890 top level 5 path <FS_TREE>/root
	ID 258 gen 641888 top level 5 path <FS_TREE>/home

`btrfs version` = Btrfs v0.20-rc1

On another system:
`btrfs subvolume list /` results in:

	ID 256 top level 5 path @
	ID 258 top level 5 path @home
	ID 261 top level 5 path @distfiles
	ID 262 top level 5 path @portage

`btrfs version` = Btrfs Btrfs v0.19

---
If the goal is to get the subvolume of the root volume, using the
contents of /proc/self/mountinfo (look for '/' in the 5th col, parse
options out of the 10th col) will work. In fact, why not just get all
the rootflags there?