freeSpan currently takes a mysterious "acct int32" argument. This is
really just a boolean and actually just needs to match the "large"
argument to alloc in order to balance out accounting.

To make this clearer, replace acct with a "large bool" argument that
must match the call to mheap.alloc.

Change-Id: Ibc81faefdf9f0583114e1953fcfb362e9c3c76de
Reviewed-on: https://go-review.googlesource.com/c/138655
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>