fuse: destroy bdi on umount

If a fuse filesystem is unmounted but the device file descriptor
remains open and a new mount reuses the old device number, then the
mount fails with EEXIST and the following warning is printed in the
kernel log:

  WARNING: at fs/sysfs/dir.c:462 sysfs_add_one+0x35/0x3d()
  sysfs: duplicate filename '0:15' can not be created

The cause is that the bdi belonging to the fuse filesystem was
destoryed only after the device file was released.  Fix this by
calling bdi_destroy() from fuse_put_super() instead.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org

fs/fuse/dev.c

@@ -281,7 +281,8 @@ __releases(&fc->lock)
 			fc->blocked = 0;
 			wake_up_all(&fc->blocked_waitq);
 		}
-		if (fc->num_background == FUSE_CONGESTION_THRESHOLD) {
+		if (fc->num_background == FUSE_CONGESTION_THRESHOLD &&
+		    fc->connected) {
 			clear_bdi_congested(&fc->bdi, READ);
 			clear_bdi_congested(&fc->bdi, WRITE);
 		}

fs/fuse/inode.c

@@ -292,6 +292,7 @@ static void fuse_put_super(struct super_block *sb)
 	list_del(&fc->entry);
 	fuse_ctl_remove_conn(fc);
 	mutex_unlock(&fuse_mutex);
+	bdi_destroy(&fc->bdi);
 	fuse_conn_put(fc);
 }
 
@@ -532,7 +533,6 @@ void fuse_conn_put(struct fuse_conn *fc)
 		if (fc->destroy_req)
 			fuse_request_free(fc->destroy_req);
 		mutex_destroy(&fc->inst_mutex);
-		bdi_destroy(&fc->bdi);
 		fc->release(fc);
 	}
 }