The fix broke mariabackup --prepare --incremental.

The restore of an incremental backup starts up (parts of) InnoDB twice.
First, all data files are discovered for applying .delta files. Then,
after the .delta files have been applied, InnoDB will be restarted
more completely, so that the redo log records will be applied via the
buffer pool.

During the first startup, the buffer pool is not initialized, and thus
trx_rseg_get_n_undo_tablespaces() must not be invoked. The apply of
the .delta files will currently assume that the --innodb-undo-tablespaces
option correctly specifies the number of undo tablespace files, just
like --backup does.

The second InnoDB startup of --prepare for applying the redo log will
properly invoke trx_rseg_get_n_undo_tablespaces().

enum srv_operation_mode: Add SRV_OPERATION_RESTORE_DELTA for
distinguishing the apply of .delta files from SRV_OPERATION_RESTORE.

srv_undo_tablespaces_init(): In mariabackup --prepare --incremental,
in the initial SRV_OPERATION_RESTORE_DELTA phase, do not invoke
trx_rseg_get_n_undo_tablespaces() because the buffer pool or the
redo logs are not available. Instead, blindly rely on the parameter
--innodb-undo-tablespaces.