btrfs/011: skip if on too small devices

btrfs/011: skip if on too small devices

btrfs/011 does require a spare device of at least 10GB, skip if we don't
have big enough devices, e.g. when testing on ramdisks.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>