Increasing the map size with the benchmark iteration count
introduced non-linearities and made benchmark runs slow when
increasing benchtime.

Rework the benchmark to use a map size independent of the
iteration count and instead re-fill it when it becomes empty.

Fixes #21546

Change-Id: Iafb6eb225e81830263f30b3aba0d449c361aec32
Reviewed-on: https://go-review.googlesource.com/57650
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>