Change mwait_bm_avoidance use to match Linux.

Even though the information is very limited, it seems the intent of
this flag is to control ACPI_BITREG_BUS_MASTER_STATUS use for C3,
not force ACPI_BITREG_ARB_DISABLE manipulations for C2, where it was
never needed, and which register not really doing anything for years.
It wasted lots of CPU time on congested global ACPI hardware lock
when many CPU cores were trying to enter/exit deep C-states same time.

On idle 80-core system it pushed ping localhost latency up to 20ms,
since badport_bandlim() via counter_ratecheck() wakes up all CPUs
same time once a second just to synchronously reset the counters.
Now enabling C-states increases the latency from 0.1 to just 0.25ms.

Discussed with: kib
MFC after: 1 month