osd: If ending on a head object get all of meta map

When ending on a head object, the head and snapshots would stay in
cleaned_meta_map until more maps arrive.  The problem as that
during a scrub an eviction could occur because scrubber.start
is already past the stray object(s) so range_intersects_scrub() is false.

Fixes: http://tracker.ceph.com/issues/23909

Signed-off-by: David Zafman <dzafman@redhat.com>

src/osd/PG.h

@@ -1601,11 +1601,15 @@ class PG : public DoutPrefixProvider {
         auto iter = cleaned_meta_map.objects.end();
         --iter; // not empty, see if clause
         auto begin = cleaned_meta_map.objects.begin();
-        while (iter != begin) {
-          auto next = iter--;
-          if (next->first.get_head() != iter->first.get_head()) {
-	    ++iter;
-	    break;
+        if (iter->first.has_snapset()) {
+          ++iter;
+        } else {
+          while (iter != begin) {
+            auto next = iter--;
+            if (next->first.get_head() != iter->first.get_head()) {
+	      ++iter;
+	      break;
+            }
           }
         }
         for_meta_scrub.objects.insert(begin, iter);