Reset the sequence counter if we observe that there is no instantaneous
page shortage.
This case can arise if we enter a shortfall of clean, inactive pages.
The PID controller will attempt to overshoot the reclamation target
because repeated scans of the inactive queue are mostly just moving
pages to the laundry queue. The laundry thread will launder pages and
move them back to the head of the inactive queue to be reclaimed, but
this does not happen immediately, so the integral term of the PID
controller grows and the page daemon tries to reclaim pages in excess of
the setpoint. However, the laundry thread will only launder enough
pages to meet the shortfall: vm_laundry_target(), which is the same as
the setpoint.
As a consequence of this, the page daemon may repeatedly fail to meet
its target, and without this seatbelt it may trigger an OOM kill.
However, we obviously shouldn't do that if we're at or above the
setpoint.