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: NJens Axboe <jens.axboe@oracle.com>

fs/fs-writeback.c

@@ -14,6 +14,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/module.h>
 #include <linux/spinlock.h>
 #include <linux/sched.h>
 #include <linux/fs.h>

include/linux/blkdev.h

 #ifndef _LINUX_BLKDEV_H
 #define _LINUX_BLKDEV_H
 
+#ifdef CONFIG_BLOCK
+
 #include <linux/sched.h>
 #include <linux/major.h>
 #include <linux/genhd.h>
@@ -32,8 +34,6 @@
 )
 #endif
 
-#ifdef CONFIG_BLOCK
-
 struct scsi_ioctl_command;
 
 struct request_queue;

include/linux/writeback.h

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

kernel/sched.c

@@ -61,6 +61,7 @@
 #include <linux/delayacct.h>
 #include <linux/reciprocal_div.h>
 #include <linux/unistd.h>
+#include <linux/pagemap.h>
 
 #include <asm/tlb.h>
 

mm/readahead.c

@@ -15,6 +15,7 @@
 #include <linux/backing-dev.h>
 #include <linux/task_io_accounting_ops.h>
 #include <linux/pagevec.h>
+#include <linux/pagemap.h>
 
 void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
 {