Avoid floating-point underflow while tracking buffer allocation rate.

When the system is idle for awhile after activity, the "smoothed_alloc"
state variable in BgBufferSync converges slowly to zero.  With standard
IEEE float arithmetic this results in several iterations with denormalized
values, which causes kernel traps and annoying log messages on some
poorly-designed platforms.  There's no real need to track such small values
of smoothed_alloc, so we can prevent the kernel traps by forcing it to zero
as soon as it's too small to be interesting for our purposes.  This issue
is purely cosmetic, since the iterations don't happen fast enough for the
kernel traps to pose any meaningful performance problem, but still it seems
worth shutting up the log messages.

The kernel log messages were previously reported by a number of people,
but kudos to Greg Matthews for tracking down exactly where they were coming
from.

src/backend/storage/buffer/bufmgr.c

@@ -1473,7 +1473,18 @@ BgBufferSync(void)
 			smoothing_samples;
 
 	/* Scale the estimate by a GUC to allow more aggressive tuning. */
-	upcoming_alloc_est = smoothed_alloc * bgwriter_lru_multiplier;
+	upcoming_alloc_est = (int) (smoothed_alloc * bgwriter_lru_multiplier);
+
+	/*
+	 * If recent_alloc remains at zero for many cycles, smoothed_alloc will
+	 * eventually underflow to zero, and the underflows produce annoying
+	 * kernel warnings on some platforms.  Once upcoming_alloc_est has gone
+	 * to zero, there's no point in tracking smaller and smaller values of
+	 * smoothed_alloc, so just reset it to exactly zero to avoid this
+	 * syndrome.  It will pop back up as soon as recent_alloc increases.
+	 */
+	if (upcoming_alloc_est == 0)
+		smoothed_alloc = 0;
 
 	/*
 	 * Even in cases where there's been little or no buffer allocation