sched_fair: Reduce latencies

Change-Id: Idded3a6cb486dc22f7c81fe43a7ed12310df25ac
writeback: Fix occasional slow sync

In case when system contains no dirty pages, wakeup_flusher_threads()
will submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
immediately without doing anything. Thus sync(1) will write all the
dirty inodes from a WB_SYNC_ALL writeback pass which is slow.

Fix the problem by using get_nr_dirty_pages() in
wakeup_flusher_threads() instead of calculating number of dirty pages
manually. That function also takes number of dirty inodes into account.

Change-Id: Iab00df48604d0f56ff7645acc51c4f8ad93b19b0