osd/: fix hobject_t construction

osd/: fix hobject_t construction

sobject_t requires only an object_t and a snapid_t.  hobject_t also
requires the hash which should be used for the object.  In most cases,
the osd must fill this in using the op message.  In cases where the hash
used does not matter (as in the metadata collection), the explicit
hobject_t(const sobject_t &) constructor supplies a hash.

Signed-off-by: Samuel Just <samuel.just@dreamhost.com>