writeback: don't propagate AOP_WRITEPAGE_ACTIVATE

This is a writeback-internal marker but we're propagating it all the way back
to userspace!.

Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -849,8 +849,10 @@ retry:
 
 			ret = (*writepage)(page, wbc, data);
 
-			if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE))
+			if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) {
 				unlock_page(page);
+				ret = 0;
+			}
 			if (ret || (--(wbc->nr_to_write) <= 0))
 				done = 1;
 			if (wbc->nonblocking && bdi_write_congested(bdi)) {