block,fs: untangle fs.h and blk_types.h

Nothing in fs.h should require blk_types.h to be included.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 797100e..e4c3870 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -10,6 +10,8 @@
 #include <linux/flex_proportions.h>
 #include <linux/backing-dev-defs.h>
 
+struct bio;
+
 DECLARE_PER_CPU(int, dirty_throttle_leaks);
 
 /*