n_page_gets is a global counter that is updated on each page access.
This also means it is updated pretty often and with a multi-core machine
it easily boils up to be the hottest counter as also reported by perf.

Using existing distributed counter framework help ease the contention
and improve the performance.

Patch also tend to increase the slot of the distributed counter from original
64 to 128 given that is new normal for next-generation machines.

The original idea and patch came from Daniel Black which is now ported
to 10.6 with some improvement and adjustment.