[fix](tablet) fix stale_version_count() returning wrong value

stale_version_count() was incorrectly returning _rs_metas.size()
instead of _stale_rs_metas.size(), causing incorrect calculation
of total version count in places like rowset_builder.cpp and
push_handler.cpp where it's used as:
  version_count() + stale_version_count()

This was effectively calculating version_count * 2 instead of
the intended total of active + stale versions.

Author: carolinchen-docker

be/src/olap/tablet_meta.h

@@ -766,7 +766,7 @@ inline size_t TabletMeta::version_count() const {
 }
 
 inline size_t TabletMeta::stale_version_count() const {
-    return _rs_metas.size();
+    return _stale_rs_metas.size();
 }
 
 inline TabletState TabletMeta::tablet_state() const {