purge_sys.stop_FTS() does not wait for purge operation
on FTS tables to finish. InnoDB DDL does purge_sys.stop_FTS()
and lock all fts tables. It eventually fails due to
n_ref_count value.

fts_stop_purge(): Stops the purge thread to process new FTS tables,
check n_ref_count of all fts index auxiliary, common tables.
This should make sure that consecutive fts_lock_tables()
is always successful.