MDEV-25683 Atomic DDL: With innodb_force_recovery=3 InnoDB: Trying to load index but the index tree has been freed

The purpose of the parameter innodb_force_recovery is to allow some
data to be dumped from a corrupted database. Its values used to be
as follows:

innodb_force_recovery=0: normal (default)

innodb_force_recovery=1: ignore (skip log for) corrupted pages or
missing data files when applying the redo log

innodb_force_recovery=2: additionally, disable background tasks
(such as the purge of committed undo logs)

innodb_force_recovery=3: additionally, disable the rollback of
recovered incomplete (not committed or XA PREPARE) transactions

innodb_force_recovery=4: same as 3 (since MDEV-19514 in MariaDB 10.5)

innodb_force_recovery=5: additionally, do not process any undo log,
disallow any writes, and force READ UNCOMMITTED isolation level

innodb_force_recovery=6: additionally, pretend that ib_logfile0 does
not exist (prevent any recovery). Never use this!

The bad thing that happens with innodb_force_recovery=3 and
innodb_force_recovery=4 is that also the rollback of any recovered
DDL transaction will be skipped. This would break the DDL log recovery
that was introduced in MDEV-17567.

For one data directory sample, the DDL log recovery would hangs due to
a conflict on the InnoDB SYS_TABLES table, because the lock holder
transaction was not rolled back due to innodb_force_recovery=3.

Fix: Make innodb_force_recovery=3 skip the DML transaction rollback only,
and make innodb_force_recovery=4 (renamed to SRV_FORCE_NO_DDL_UNDO)
behave like innodb_force_recovery=3 used to (skip the rollback of all
recovered transactions, both DML and DDL).

Startup with innodb_force_recovery=4 will be unaffected by this change.
(There may be hangs, possibly preceded by messages about failing to
load an index.)

Side note: With innodb_force_recovery=5, any DDL log for InnoDB tables
will be essentially ignored by InnoDB, but the server will start up.