Btrfs: don't return congestion in write_cache_pages as often

On fast devices that go from congested to uncongested very quickly, pdflush
is waiting too often in congestion_wait, and the FS is backing off to
easily in write_cache_pages.

For now, fix this on the btrfs side by only checking congestion after
some bios have already gone down.  Longer term a real fix is needed
for pdflush, but that is a larger project.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -2377,11 +2377,6 @@ static int extent_write_cache_pages(struct extent_io_tree *tree,
 	int scanned = 0;
 	int range_whole = 0;
 
-	if (wbc->nonblocking && bdi_write_congested(bdi)) {
-		wbc->encountered_congestion = 1;
-		return 0;
-	}
-
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
 		index = mapping->writeback_index; /* Start from prev offset */