if the rootfs is a btrfs subvolume, delete it instead of rm -rf
if the rootfs is a btrfs subvolume, delete it instead of rm -rf

Check if the rootfs is btrfs subvolume, and if so delete it. Otherwise
fall back to the rm -rf.

Signed-off-by: Dwight Engen <1793d65cf7b9eefb2bb8f5bee9a4b8ba782c26da@oracle.com>

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