osd: take ondisk_read_lock on src_oids

osd: take ondisk_read_lock on src_oids

We need to take the ondisk read lock on src oids for multiobject operations
(like clonerange) to ensure that written data has hit disk before we
clone it elsewhere.

Order of acquisition doesn't actually matter here, since the ondisk locks
are all leaves in the lock dependency hierarchy.

Signed-off-by: Sage Weil <sage@newdream.net>