Optimization in buf_pool_get_oldest_modification if srv_buf_pool_instances == 1

[Laurynas Biveinis]

buf_pool_get_oldest_modification() loops over all buffer pool instances, acquiring the flush list mutex for each of them.

Before the loop it enters the log flush order mutex:

 /* When we traverse all the flush lists we don't want another
 thread to add a dirty page to any flush list. */
 log_flush_order_mutex_enter();

In the case of srv_buf_pool_instances == 1 this is unnecessary as the flush list mutex will take care of it

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2418