crush: add root_bucket to identify underfull buckets

crush: add root_bucket to identify underfull buckets

All underfull buckets under root_buckets will be taken as target

For the crule rule:
    step take datacenter0
    step chooseleaf firstn 2 type host
    step emit
    step take datacenter1
    step chooseleaf firstn 2 type host
    step emit

If one host contains overfull osd but no underfull osd,
it will use other underfull buckets as target, which
maybe not in the same datacenter, that will
broke the rule.

Fixes: http://tracker.ceph.com/issues/38826
Signed-off-by: huangjun <huangjun@xsky.com>