crimson/osd: put snapmapper's key-value pairs into dedicated objs

Otherwise, PG::read_log_and_missing() will meet those key-values and
won't know what to do with them. This is modeling what the classic
osd is doing

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>

src/crimson/osd/pg.cc

@@ -145,7 +145,7 @@ PG::PG(
     osdriver(
       &shard_services.get_store(),
       coll_ref,
-      pgid.make_pgmeta_oid()),
+      make_snapmapper_oid()),
     snap_mapper(
       this->shard_services.get_cct(),
       &osdriver,
@@ -610,6 +610,8 @@ void PG::init(
   peering_state.init(
     role, newup, new_up_primary, newacting,
     new_acting_primary, history, pi, t);
+  assert(coll_ref);
+  t.touch(coll_ref->get_cid(), make_snapmapper_oid());
 }
 
 seastar::future<> PG::read_state(crimson::os::FuturizedStore::Shard* store)

src/crimson/osd/pg.h

@@ -632,6 +632,12 @@ class PG : public boost::intrusive_ref_counter<
   OSDriver osdriver;
   SnapMapper snap_mapper;
 
+  ghobject_t make_snapmapper_oid() {
+    return ghobject_t(hobject_t(
+      sobject_t(
+       object_t("snapmapper"),
+       0)));
+  }
 public:
   // PeeringListener
   void publish_stats_to_osd() final;