fuse: fix bdi naming conflict

Fuse allocates a separate bdi for each filesystem, and registers them
in sysfs with "MAJOR:MINOR" of sb->s_dev (st_dev). This works fine for
anon devices normally used by fuse, but can conflict with an already
registered BDI for "fuseblk" filesystems, where sb->s_dev represents a
real block device. In particularl this happens if a non-partitioned
device is being mounted.

Fix by registering with a different name for "fuseblk" filesystems.

Thanks to Ioan Ionita for the bug report.

Signed-off-by: Miklos Szeredi <[e-mail]>
Reported-by: Ioan Ionita <[e-mail]>
Tested-by: Ioan Ionita <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/ABI/testing/sysfs-class-bdi

···
 	non-block filesystems which provide their own BDI, such as NFS
 	and FUSE.
 
+MAJOR:MINOR-fuseblk
+
+	Value of st_dev on fuseblk filesystems.
+
 default
 
 	The default backing dev, used for non-block device backed

fs/fuse/inode.c

···
 		err = bdi_init(&fc->bdi);
 		if (err)
 			goto error_kfree;
-		err = bdi_register_dev(&fc->bdi, fc->dev);
+		if (sb->s_bdev) {
+			err = bdi_register(&fc->bdi, NULL, "%u:%u-fuseblk",
+					   MAJOR(fc->dev), MINOR(fc->dev));
+		} else {
+			err = bdi_register_dev(&fc->bdi, fc->dev);
+		}
 		if (err)
 			goto error_bdi_destroy;
 		/*