btrfs: fix mount and ioctl device scan ioctl race

Technically this extends the critical section covered by uuid_mutex to:

- parse early mount options -- here we can call device scan on paths
  that can be passed as 'device=/dev/...'

- scan the device passed to mount

- open the devices related to the fs_devices -- this increases
  fs_devices::opened

The race can happen when mount calls one of the scans and there's
another one called eg. by mkfs or 'btrfs dev scan':

Mount                                  Scan
-----                                  ----
scan_one_device (dev1, fsid1)
                                       scan_one_device (dev2, fsid1)
				           add the device
					   free stale devices
					       fsid1 fs_devices::opened == 0
					           find fsid1:dev1
					           free fsid1:dev1
					           if it's the last one,
					            free fs_devices of fsid1
						    too

open_devices (dev1, fsid1)
   dev1 not found

When fixed, the uuid mutex will make sure that mount will increase
fs_devices::opened and this will not be touched by the racing scan
ioctl.

Reported-and-tested-by: syzbot+909a5177749d7990ffa4@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+ceb2606025ec1cc3479c@syzkaller.appspotmail.com
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -1555,19 +1555,19 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 
 	mutex_lock(&uuid_mutex);
 	error = btrfs_parse_early_options(data, mode, fs_type, &fs_devices);
-	mutex_unlock(&uuid_mutex);
-	if (error)
+	if (error) {
+		mutex_unlock(&uuid_mutex);
 		goto error_fs_info;
+	}
 
-	mutex_lock(&uuid_mutex);
 	error = btrfs_scan_one_device(device_name, mode, fs_type, &fs_devices);
-	mutex_unlock(&uuid_mutex);
-	if (error)
+	if (error) {
+		mutex_unlock(&uuid_mutex);
 		goto error_fs_info;
+	}
 
 	fs_info->fs_devices = fs_devices;
 
-	mutex_lock(&uuid_mutex);
 	error = btrfs_open_devices(fs_devices, mode, fs_type);
 	mutex_unlock(&uuid_mutex);
 	if (error)