osd: ScrubMap::object::get_object_info_t helper

Encapsulate decoding of attrs[OI_ATTR] in a helper and return on error
when OI_ATTR is not found. PG::repair_object uses the helper, logs an
error if it fails and return immediately.

http://tracker.ceph.com/issues/8345 Fixes: #8345

Signed-off-by: Loic Dachary <loic@dachary.org>