MDEV-18952 CHECK TABLE should use READ UNCOMMITED if innodb_force_recovery>=5

MDEV-15418 changed InnoDB to use the READ UNCOMMITTED isolation level
when the transaction recovery is disabled by setting
innodb_force_recovery to 5 or 6. Alas, CHECK TABLE would still
internally use REPEATABLE READ. If the server was started without
purge running into completion (and resetting all DB_TRX_ID thanks
to MDEV-12288), this would cause errors about any DB_TRX_ID that
had not been reset to 0.

storage/innobase/handler/ha_innodb.cc

@@ -14512,8 +14512,11 @@ ha_innobase::check(
 	/* We must run the index record counts at an isolation level
 	>= READ COMMITTED, because a dirty read can see a wrong number
 	of records in some index; to play safe, we use always
-	REPEATABLE READ here */
-	m_prebuilt->trx->isolation_level = TRX_ISO_REPEATABLE_READ;
+	REPEATABLE READ here (except when undo logs are unavailable) */
+	m_prebuilt->trx->isolation_level = srv_force_recovery
+		>= SRV_FORCE_NO_UNDO_LOG_SCAN
+		? TRX_ISO_READ_UNCOMMITTED
+		: TRX_ISO_REPEATABLE_READ;
 
 	ut_ad(!m_prebuilt->table->corrupted);