fs-writeback.c: bitfields should be unsigned

This fixes sparse noise:
  error: dubious one-bit signed bitfield
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/fs-writeback.c

@@ -42,9 +42,9 @@ struct wb_writeback_args {
 	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;
 };
 
 /*