tools/power turbostat: Fix offset overflow issue in index converting

In the Linux kernel, the following vulnerability has been resolved:

tools/power turbostat: Fix offset overflow issue in index converting

The idx_to_offset() function returns type int (32-bit signed), but

MSR_PKG_ENERGY_STAT is u32 and would be interpreted as a negative number.

The end result is that it hits the if (offset < 0) check in update_msr_sum()

which prevents the timer callback from updating the stat in the background when

long durations are used. The similar issue exists in offset_to_idx() and

update_msr_sum(). Fix this issue by converting the 'int' to 'off_t' accordingly.