Btrfs: define write_cache_pages for linux kernel <= 2.6.20 instead

write_cache_pages doesn't exist in linux 2.6.20,  change the #if
condition to match that.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -2109,8 +2109,7 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
 	return 0;
 }
 
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-
+#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,20)
 /* Taken directly from 2.6.23 for 2.6.18 back port */
 typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
                                 void *data);