Silence bogus GCC 7 warnings -Wimplicit-fallthrough

Do not silence uncertain cases, or fix any bugs.

The only functional change should be that ha_federated::extra()
is not calling DBUG_PRINT to report an unhandled case for
HA_EXTRA_PREPARE_FOR_DROP.

storage/maria/ma_extra.c

@@ -157,6 +157,7 @@ int maria_extra(MARIA_HA *info, enum ha_extra_function function,
     if (info->s->data_file_type != DYNAMIC_RECORD)
       break;
     /* Remove read/write cache if dynamic rows */
+    /* fall through */
   case HA_EXTRA_NO_CACHE:
     if (info->opt_flag & (READ_CACHE_USED | WRITE_CACHE_USED))
     {
@@ -313,7 +314,7 @@ int maria_extra(MARIA_HA *info, enum ha_extra_function function,
     share->state.open_count= 1;
     share->changed= 1;
     _ma_mark_file_changed_now(share);
-    /* Fall trough */
+    /* Fall through */
   case HA_EXTRA_PREPARE_FOR_RENAME:
   {
     my_bool do_flush= MY_TEST(function != HA_EXTRA_PREPARE_FOR_DROP);

storage/maria/ma_recovery.c

@@ -3061,7 +3061,7 @@ static MARIA_HA *get_MARIA_HA_from_REDO_record(const
   case LOGREC_REDO_INDEX:
   case LOGREC_REDO_INDEX_FREE_PAGE:
     index_page_redo_entry= 1;
-    /* Fall trough*/
+    /* Fall through */
   case LOGREC_REDO_INSERT_ROW_HEAD:
   case LOGREC_REDO_INSERT_ROW_TAIL:
   case LOGREC_REDO_PURGE_ROW_HEAD: