writeback: introduce writeback_control.inodes_written

The flusher works on dirty inodes in batches, and may quit prematurely
if the batch of inodes happen to be metadata-only dirtied: in this case
wbc->nr_to_write won't be decreased at all, which stands for "no pages
written" but also mis-interpreted as "no progress".

So introduce writeback_control.inodes_written to count the inodes get
cleaned from VFS POV.  A non-zero value means there are some progress on
writeback, in which case more writeback can be tried.

Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>