fs-writeback.c: bitfields should be unsigned

This fixes sparse noise:
  error: dubious one-bit signed bitfield

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 4b37f7c..24e85ce 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -42,9 +42,9 @@
 	long nr_pages;
 	struct super_block *sb;
 	enum writeback_sync_modes sync_mode;
-	int for_kupdate:1;
-	int range_cyclic:1;
-	int for_background:1;
+	unsigned int for_kupdate:1;
+	unsigned int range_cyclic:1;
+	unsigned int for_background:1;
 };
 
 /*