osd: expose bytes used/avail via perf / asok

This values are already sent to the monitor.  Expose them via the admin
socket too so collectd/diamond/whatever can pick them up.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #569 from ceph/wip-osd-stat

osd: expose bytes used/avail via perf / asok