btrfs: test orphan item cleanup

btrfs: test orphan item cleanup

This test tests three conditions where orphan items need to be
cleaned up:

1) Default subvolume is fs tree root (mkfs default)
2) Default subvolume has been set explicitly to another subvolumei
   (set-default)
3) Looking up a subvolume during a traversal

In the kernel, these three cases use separate mechanisms to look up
the subvolumes. There was a bug in the kernel where case #2 wasn't
getting handled and so orphan items would never be cleaned up if the
default subvolume was explicitly set. This issue was fixed with the
upstream commit: btrfs: cleanup orphans while looking up default
subvolume.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>