write-back: fix nr_to_write counter

Commit 05fe478dd04e02fa230c305ab9b5616669821dd3 introduced some
@wbc->nr_to_write breakage.

It made the following changes:
 1. Decrement wbc->nr_to_write instead of nr_to_write
 2. Decrement wbc->nr_to_write _only_ if wbc->sync_mode == WB_SYNC_NONE
 3. If synced nr_to_write pages, stop only if if wbc->sync_mode ==
    WB_SYNC_NONE, otherwise keep going.

However, according to the commit message, the intention was to only make
change 3.  Change 1 is a bug.  Change 2 does not seem to be necessary,
and it breaks UBIFS expectations, so if needed, it should be done
separately later.  And change 2 does not seem to be documented in the
commit message.

This patch does the following:
 1. Undo changes 1 and 2
 2. Add a comment explaining change 3 (it very useful to have comments
    in _code_, not only in the commit).

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -1051,13 +1051,22 @@ continue_unlock:
 				}
  			}
 
-			if (wbc->sync_mode == WB_SYNC_NONE) {
-				wbc->nr_to_write--;
-				if (wbc->nr_to_write <= 0) {
-					done = 1;
-					break;
-				}
+			if (nr_to_write > 0)
+				nr_to_write--;
+			else if (wbc->sync_mode == WB_SYNC_NONE) {
+				/*
+				 * We stop writing back only if we are not
+				 * doing integrity sync. In case of integrity
+				 * sync we have to keep going because someone
+				 * may be concurrently dirtying pages, and we
+				 * might have synced a lot of newly appeared
+				 * dirty pages, but have not synced all of the
+				 * old dirty pages.
+				 */
+				done = 1;
+				break;
 			}
+
 			if (wbc->nonblocking && bdi_write_congested(bdi)) {
 				wbc->encountered_congestion = 1;
 				done = 1;