rgw: init some manifest fields when handling explicit objs

Fixes: #11455
When dealing with old manifest that has explicit objs, we also
need to set the head size and head object correctly so that
code that relies on this info doesn't break.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit d7bf8cb)

src/rgw/rgw_rados.h

@@ -327,6 +327,12 @@ class RGWObjManifest {
       ::decode(rules, bl);
     } else {
       explicit_objs = true;
+      if (!objs.empty()) {
+        map<uint64_t, RGWObjManifestPart>::iterator iter = objs.begin();
+        head_obj = iter->second.loc;
+        head_size = iter->second.size;
+        max_head_size = head_size;
+      }
     }
 
     if (struct_v >= 4) {