Before this patch, InnoDB purge submitted innodb_purge_threads-1 tasks,
even if there was insufficient work for all. For example,
with innodb_purge_threads at the default (4) and updates on a single table, only one thread could work,
yet three tasks were submitted. This caused unnecessary wakeups and context
switches.

The patch modifies trx_purge_attach_undo_recs() to create as many work
items as necessary. The purge coordinator now processes more tasks itself,
which reduces context switching and improves efficiency.