Prior to r254304, we only began scanning the active page queue when the
amount of free memory was close to the point at which we would begin
reclaiming pages.  Now, we continuously scan the active page queue,
regardless of the amount of free memory.  Consequently, we are continuously
calling pmap_ts_referenced() on active pages.

Prior to this change, pmap_ts_referenced() would always demote superpage
mappings in order to obtain finer-grained reference information.  This made
sense because we were coming under memory pressure and would soon have to
begin reclaiming pages.  Now, however, with continuous scanning of the
active page queue, these demotions are taking a toll on performance.  For
example, on one of my test machines, the running time for the HPCC Random
Access benchmark (also known as GUPS) has increased by 54%.  To address this
problem, I have replaced the demotion with a heuristic for periodically
clearing the reference flag on superpage mappings.

Reviewed by:	kib
Approved by:	re (glebius)
Sponsored by:	EMC / Isilon Storage Division