initmpfs: move bdi setup from init_rootfs to init_ramfs

Even though ramfs hasn't got a backing device, commit e0bf68dd ("mm:
bdi init hooks") added one anyway, and put the initialization in
init_rootfs() since that's the first user, leaving it out of init_ramfs()
to avoid duplication.

But initmpfs uses init_tmpfs() instead, so move the init into the
filesystem's init function, add a "once" guard to prevent duplicate
initialization, and call the filesystem init from rootfs init.

This goes part of the way to allowing ramfs to be built as a module.

[akpm@linux-foundation.org; using bit 1 was odd]
Signed-off-by: Rob Landley <rob@landley.net>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jim Cromie <jim.cromie@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ramfs/inode.c

@@ -249,7 +249,7 @@ static struct dentry *rootfs_mount(struct file_system_type *fs_type,
 {
 	static unsigned long once;
 
-	if (test_and_set_bit(1, &once))
+	if (test_and_set_bit(0, &once))
 		return ERR_PTR(-ENODEV);
 
 	return mount_nodev(fs_type, flags, data, ramfs_fill_super);
@@ -275,21 +275,34 @@ static struct file_system_type rootfs_fs_type = {
 
 static int __init init_ramfs_fs(void)
 {
-	return register_filesystem(&ramfs_fs_type);
+	static unsigned long once;
+	int err;
+
+	if (test_and_set_bit(0, &once))
+		return 0;
+
+	err = bdi_init(&ramfs_backing_dev_info);
+	if (err)
+		return err;
+
+	err = register_filesystem(&ramfs_fs_type);
+	if (err)
+		bdi_destroy(&ramfs_backing_dev_info);
+
+	return err;
 }
 module_init(init_ramfs_fs)
 
 int __init init_rootfs(void)
 {
-	int err;
+	int err = register_filesystem(&rootfs_fs_type);
 
-	err = bdi_init(&ramfs_backing_dev_info);
 	if (err)
 		return err;
 
-	err = register_filesystem(&rootfs_fs_type);
+	err = init_ramfs_fs();
 	if (err)
-		bdi_destroy(&ramfs_backing_dev_info);
+		unregister_filesystem(&rootfs_fs_type);
 
 	return err;
 }