MDEV-5010: InnoDB errors appearing in logs with upgrade from 10.0.0 to 10.0.4.

Analysis: In earlier MySQL 5.6 versions this table innodb_index_stats  used to have a foreign key referencing to innodb_table_stats. However, in newer MySQL 5.6 versions this foreign key is removed and if you upgrade, your innodb_table_stats is created by the earlier mariadb version, thus a newer version will complain that the table is incorrectly defined.

Added drop foreign key on mysql_system_tables_fix.sql to be executed on mysql_upgrade.

scripts/mysql_system_tables_fix.sql

@@ -664,5 +664,12 @@ alter table tables_priv  modify Grantor      char(141) COLLATE utf8_bin not null
 # This should not be needed, but gives us some extra testing that the above
 # changes was correct
 
+set @have_innodb= (select count(engine) from information_schema.engines where engine='INNODB' and support != 'NO');
+SET @innodb_index_stats_fk= (select count(*) from information_schema.referential_constraints where constraint_schema='mysql' and table_name = 'innodb_index_stats' and referenced_table_name = 'innodb_table_stats' and constraint_name = 'innodb_index_stats_ibfk_1');
+SET @str=IF(@innodb_index_stats_fk > 0 and @have_innodb > 0, "ALTER TABLE mysql.innodb_index_stats DROP FOREIGN KEY `innodb_index_stats_ibfk_1`", "SET @dummy = 0");
+PREPARE stmt FROM @str;
+EXECUTE stmt;
+DROP PREPARE stmt; 
+
 flush privileges;