btrfs/085: check flakey device instead of backend device

btrfs/085: check flakey device instead of backend device

btrfs/085 mount and works on $FLAKEY_DEV, but it's inspecting the
underlying $SCRATCH_DEV. Since writing to dm-flakey does not
invalidate page cache of $SCRATCH_DEV, the btrfs command can read a
stalled data from page cache. We should check the flakey device to
avoid such cache inconsistency.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>