btrfs: refactor locked condition in btrfs_init_new_device

Invert unlocked to locked and exploit the fact it can only ever be
modified if we are adding a new device to a seed filesystem. This allows
to simplify the check in error: label. No semantics changes.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>