congestion_wait() mean "waiting quueue congestion is cleared".
That said, if the system have plenty dirty pages and flusher thread push
new request to IO queue conteniously, IO queue are not cleared
congestion status for long time. thus, congestion_wait(HZ/10) become
almostly equivalent schedule_timeout(HZ/10).
However, synchronous lumpy reclaim donesn't need this
congestion_wait() at all. shrink_page_list(PAGEOUT_IO_SYNC) are
using wait_on_page_writeback() and it provide sufficient waiting.
Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>