MDEV-25936 Crash during DDL that involves FULLTEXT INDEX

In commit 1bd681c we introduced
a work-around for the missing MDL protection when the internal
tables of FULLTEXT INDEX are dropped during DDL operations.
That work-around suffered from a race condition. A purge thread
could have narrowly passed purge_sys.check_stop_FTS() and then
(while holding dict_sys.mutex) acquire a table reference right
after fts_lock_table() determined that no references were being
held.

fts_lock_table(): Protect the reference check with dict_sys.mutex.

Thanks to Thirunarayanan Balathandayuthapani for repeating the
failure and testing the fix.

storage/innobase/fts/fts0fts.cc

@@ -1549,15 +1549,19 @@ static dberr_t fts_lock_table(trx_t *trx, const char *table_name)
   {
     dberr_t err= lock_table_for_trx(table, trx, LOCK_X);
     /* Wait for purge threads to stop using the table. */
+    dict_sys.mutex_lock();
     for (uint n= 15; table->get_ref_count() > 1; )
     {
+      dict_sys.mutex_unlock();
       if (!--n)
       {
         err= DB_LOCK_WAIT_TIMEOUT;
         break;
       }
       std::this_thread::sleep_for(std::chrono::milliseconds(50));
+      dict_sys.mutex_lock();
     }
+    dict_sys.mutex_unlock();
     table->release();
     return err;
   }