This is a backport of 161e4bfa.

trans_rollback_to_savepoint(): Only release metadata locks (MDL)
if the storage engines agree, after the changes were already rolled back.

Ever since commit 3792693f
and mysql/mysql-server@55ceedbc3feb911505dcba6cee8080d55ce86dda
we used to cheat here and always release MDL if the binlog is disabled.

MDL are supposed to prevent race conditions between DML and DDL also
when no replication is in use. MDL are supposed to be a superset of
InnoDB table locks: InnoDB table lock may only exist if the thread
also holds MDL on the table name.

In the included test case, ROLLBACK TO SAVEPOINT would wrongly release
the MDL on both tables and let ALTER TABLE proceed, even though the DML
transaction is actually holding locks on the table.

Until commit 1bd681c8 (MDEV-25506)
in MariaDB 10.6, InnoDB would often work around the locking violation
in a blatantly non-ACID way: If locks exist on a table that is being
dropped (in this case, actually a partition of a table that is being
rebuilt by ALTER TABLE), InnoDB could move the table (or partition)
into a queue, to be dropped after the locks and references had been
released. If the lock is not released and the original copy of the
table not dropped quickly enough, a name conflict could occur on
a subsequent ALTER TABLE.

The scenario of commit 3792693f
is unaffected by this fix, because mysqldump
would use non-locking reads, and the transaction would not be holding
any InnoDB locks during the execution of ROLLBACK TO SAVEPOINT.
MVCC reads inside InnoDB are only covered by MDL and page latches,
not by any table or record locks.

FIXME: It would be nice if storage engines were specifically asked
which MDL can be released, instead of only offering a choice
between all or nothing. InnoDB should be able to release any
locks for tables that are no longer in trx_t::mod_tables, except
if another transaction had converted some implicit record locks
to explicit ones, before the ROLLBACK TO SAVEPOINT had been completed.

Reviewed by: Sergei Golubchik