btrfs: drop unused parameter in mount_subvol

@device_name in mount_subvol() is not used, drop it.  Also see:
5bedc48a

 ("btrfs: drop unused parameters from mount_subvol").

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