OSD: update_osd_stat take heartbeat_lock
OSD: update_osd_stat take heartbeat_lock

Previously update_osd_stat had a race with code modifying heartbeat_from
causing the iterator increment to occasionally segfault.

Signed-off-by: Samuel Just <4c46b94684b338bae1ed5e7e1321718dd75401cc@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph