btrfs: test for device dynamic rescan

btrfs: test for device dynamic rescan

Make sure missing device is included in the alloc list when it is
scanned on a mounted FS.

This test case needs btrfs kernel patch which is in the ML
  [PATCH] btrfs: handle dynamically reappearing missing device

Without the kernel patch, the test will run, but reports as
failed, as the device scanned won't appear in the alloc_list.

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