Fix race condition in TSTabletManager::GenerateFullTabletReport

Summary:
dirty_tablets_ is accessed w/o lock.
Fixed that.

Test Plan: ybd tsan --cxx-test integration-tests_registration-test --gtest_filter RegistrationTest.TestTabletReports -n 100

Reviewers: mikhail, amitanand, timur

Reviewed By: timur

Subscribers: ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D4743

src/yb/tserver/ts_tablet_manager.cc

@@ -1233,6 +1233,8 @@ void TSTabletManager::GenerateFullTabletReport(TabletReportPB* report) {
   for (const auto& replica : to_report) {
     CreateReportedTabletPB(replica, report->add_updated_tablets());
   }
+
+  std::lock_guard<RWMutex> l(lock_);
   dirty_tablets_.clear();
 }