btrfs/124: add balance --full-balance option

btrfs balance needs --full-balance option since 4.6, so check the
version and then use it.

As this may be useful for other btrfs tests as well, so this patch
also adds _run_btrfs_balance_start() to the common/btrfs file.

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