runtime: refactor hmap.extra.overflow array into two separate fields
This makes it easier to deduce from the field names which overflow
field corresponds to h.buckets and which to h.oldbuckets by aligning
the naming with the buckets fields in hmap.
Change-Id: I8d6a729229a190db0212bac012ead1a3c13cf5d0
Reviewed-on: https://go-review.googlesource.com/62411
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>