btrfs: test RAID5 device reappear and balance

btrfs: test RAID5 device reappear and balance

The test does the following:
Initialize a RAID5 with some data

Re-mount RAID5 degraded with _dev3_ missing and write data.
Save md5sum checkpoint1

Re-mount healthy RAID5

Let balance fix degraded blocks.
Save md5sum checkpoint2

Re-mount RAID1 degraded now with _dev1_ missing.
Save md5sum checkpoint3

Verify if all three md5sum matches

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>