MDEV-24258 fixup: Throttle purge with exclusive dict_sys.latch

A performance regression was observed after
commit 82b7c561
because purge tasks would end up waiting more elsewhere,
most notably trx_purge_get_next_rec() and trx_purge_truncate_history().

row_purge_parse_undo_rec(): Prevent the performance regression by
unnecessarily acquiring dict_sys.latch in exclusive mode after the
table lookup.

storage/innobase/row/row0purge.cc

@@ -1038,6 +1038,12 @@ row_purge_parse_undo_rec(
 		goto err_exit;
 	}
 
+	/* FIXME: We are acquiring exclusive dict_sys.latch only to
+	avoid increased wait times in
+	trx_purge_get_next_rec() and trx_purge_truncate_history(). */
+	dict_sys.lock(SRW_LOCK_CALL);
+	dict_sys.unlock();
+
 already_locked:
 	ut_ad(!node->table->is_temporary());