btrfs: test RAID1 device reappear and balance

btrfs: test RAID1 device reappear and balance

The test does the following:
  Initialize a RAID1 with some data

  Re-mount RAID1 degraded with _dev1_ and write up to
  half of the FS capacity
  Save md5sum checkpoint1

  Re-mount healthy RAID1

  Let balance re-silver.
  Save md5sum checkpoint2

  Re-mount RAID1 degraded with _dev2_
  Save md5sum checkpoint3

  Verify if all three md5sum match

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