btrfs/022: add a quota rescan -w to wait rescan finished.

btrfs/022: add a quota rescan -w to wait rescan finished.

When we enable quota, btrfs will rescan quota numbers. We need
to wait the rescan finished before any more operations on btrfs
qgroups. Otherwith, the new btrfs-progs would WARN out:

WARNING: Rescan is running, qgroup data may be incorrect.

It would make btrfs/022 failed.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>