Fix warnings with !CONFIG_BLOCK

Hide everything in blkdev.h with CONFIG_BLOCK isn't set, and fixup
the (few) files that fail to build because they were relying on blkdev.h
pulling in extra includes for them.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/writeback.h

@@ -5,6 +5,7 @@
 #define WRITEBACK_H
 
 #include <linux/sched.h>
+#include <linux/fs.h>
 
 struct backing_dev_info;