crush: add adjust_subtree_weight()

Adjust the weight for all (leaf) items beneath a subtree.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: add 'osd crush reweight-subtree ...'

Reweight all items within a subtree.  For example, you might reweigh tall
OSDs within a rack to a new value.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: simplify 'osd crush reweight ...' flow

Use the goto labels; drop the do/while.

Signed-off-by: Sage Weil <sage@inktank.com>