LRUItr::start(): fix inverted reset condition

Port: https://github.com/percona/percona-server/commit/dc344fba7e3d272ac18e8d6589b37c678f1e1ad5

See: https://perconadev.atlassian.net/browse/PS-11446

This bug causes the LRU scan to start more likely from the LRU tail (oldest-old element), rather than resuming from the point in the old-sublist where the scan previously stopped.
Fixing this will ease scanning progress in the old-sublist, with potential benefits for page evictions when threads are synchronously executing buf_LRU_scan_and_free_block() from buf_LRU_get_free_block() because buf_pool.free list is empty and buf_pool.try_LRU_scan is enabled.