MDEV-5949: Performance of XtraDB slows down significantly on long benchmarks
when compressed tables are used.

Analysis: Number of flushed pages is incorrectly calculated at
buf_do_LRU_batch. This leads to problem when utility function
flushes dirty blocks from the end of the flush list of
all buffer pool instances in a loop until enough pages are flushed
or time limit is reached. As number of flushed pages is incorrectly
calculated, the loop mostly try to flush until time limit is
reached because the number of pages limit is not reached.

Fix: Fix the calculation of flushed pages (very short). This fix
was provided by Alexey Stroganov (Percona).