crush: fix incorrect use of adjust_item_weight method

adjust_item_weight method will adjust all buckets which the item
inside. If the osd.0 in host=fake01 and host=fake02, we execute
"ceph osd crush osd.0 10 host=fake01", it not only will adjust fake01's
weight, but also will adjust fake02's weight.

the patch add adjust_item_weightf_in_loc method and fix remove_item,
_remove_item_under, update_item, insert_item, detach_bucket methods.

Signed-off-by: Rongze Zhu <zrzhit@gmail.com>
Merge pull request #2689 from zhurongze/fix-crush

crush: fix incorrect use of adjust_item_weight method

Reviewed-by: Sage Weil <sage@redhat.com>