btrfs: remove checks for arg argument in btrfs_ioctl_balance

With the removal of balance v1 ioctl the 'arg' argument is guaranteed to
be present so simply remove all conditional code which checks for its
presence.

Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>