osd: fix theoretical use-after-free of OSDMap

In practice, the map will remain pinned for a while, but this
will make coverity happy.

*** CID 1231685:  Use after free  (USE_AFTER_FREE)
/osd/OSD.cc: 6223 in OSD::handle_osd_map(MOSDMap *)()
6217
6218           if (o->test_flag(CEPH_OSDMAP_FULL))
6219            last_marked_full = e;
6220           pinned_maps.push_back(add_map(o));
6221
6222           bufferlist fbl;
>>>     CID 1231685:  Use after free  (USE_AFTER_FREE)
>>>     Calling "encode" dereferences freed pointer "o".
6223           o->encode(fbl);
6224
6225           hobject_t fulloid = get_osdmap_pobject_name(e);
6226           t.write(coll_t::META_COLL, fulloid, 0, fbl.length(), fbl);
6227           pin_map_bl(e, fbl);
6228           continue;

Signed-off-by: Sage Weil <sage@redhat.com>