init: Initialize noop_backing_dev_info early

noop_backing_dev_info is used by superblocks of various
pseudofilesystems such as kdevtmpfs. After commit 10e14073107d
("writeback: Fix inode->i_io_list not be protected by inode->i_lock
error") this broke because __mark_inode_dirty() started to access more
fields from noop_backing_dev_info and this led to crashes inside
locked_inode_to_wb_and_lock_list() called from __mark_inode_dirty().
Fix the problem by initializing noop_backing_dev_info before the
filesystems get mounted.

Fixes: 10e14073107d ("writeback: Fix inode->i_io_list not be protected by inode->i_lock error")
Reported-and-tested-by: Suzuki K Poulose <[e-mail]>
Reported-and-tested-by: Alexandru Elisei <[e-mail]>
Reported-and-tested-by: Guenter Roeck <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

drivers/base/init.c

···
 #include <linux/init.h>
 #include <linux/memory.h>
 #include <linux/of.h>
+#include <linux/backing-dev.h>
 
 #include "base.h"
 
···
 void __init driver_init(void)
 {
 	/* These are the core pieces */
+	bdi_init(&noop_backing_dev_info);
 	devtmpfs_init();
 	devices_init();
 	buses_init();

include/linux/backing-dev.h

···
 
 extern struct backing_dev_info noop_backing_dev_info;
 
+int bdi_init(struct backing_dev_info *bdi);
+
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @wb: bdi_writeback of interest

mm/backing-dev.c

···
 }
 postcore_initcall(bdi_class_init);
 
-static int bdi_init(struct backing_dev_info *bdi);
-
 static int __init default_bdi_init(void)
 {
-	int err;
-
 	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_UNBOUND |
 				 WQ_SYSFS, 0);
 	if (!bdi_wq)
 		return -ENOMEM;
-
-	err = bdi_init(&noop_backing_dev_info);
-
-	return err;
+	return 0;
 }
 subsys_initcall(default_bdi_init);
 
···
 
 #endif	/* CONFIG_CGROUP_WRITEBACK */
 
-static int bdi_init(struct backing_dev_info *bdi)
+int bdi_init(struct backing_dev_info *bdi)
 {
 	int ret;